1) Whoever makes between himself and Allah(swt) intermediaries and relies upon them (tawakkul) becomes a Mushrik ( polytheist).
A common example would be if a husband and his wife go to a peer (religious figure) or a saint and request him to ‘bless’ them with a child, wrongly believing that the man is the one who will enable them to have a child. Allah Almighty says: “Those on whom you call besides Allah are only slaves like yourselves.” (al A’raaf 7:194)

Something, which is less obvious but is widespread among the Muslim Ummah worldwide is the use of charms and amulets (taweez). A person may wear an amulet around his neck or arm and expect it to cure him of his illness. The belief in charms gives created things the Divine Power to avert misfortune and thus, those who subscribe to such beliefs in effect try to take what belongs to Allah and assign it to Allah's creation. Therefore, the belief in charms is an evident form of shirk.

Allah is the only real power in existence, it is He who gives all things the power to move and change. Nothing can happen except by the will of Allah. In recognition of this reality Prophet Muhammad (sallallahu ‘alaihi wa sallam) used to often repeat “La hawla wa laa quwwata illaa billaah” (There is no movement nor power except by Allah’s will). Allah (SWT) says: “If Allah allows harm to befall you none can remove it except Him.” (al Ana’m 6:17)

Also:
“And no calamity strikes except by Allah’s permission” (at Taghaabun 64:11)

Once we understand that nothing happens except by the will of Allah(SWT), and that all good is from Allah, and no harm can befall us or be removed from us except by the will of Allah, then it is clear and easy to understand that we should not believe that peers (religious, holy people), charms or amulets can bring us good or remove harm.

We recite at least 17 times in our prayers: “You (alone) we worship, and from You (alone) we seek help”. Surah Al-Fatiha 1, Ayah 5. Therefore every Muslim should implement these verses in real life and not merely recite them. Allah(SWT) says: “When my servents ask you (O Muhammad) about Me (tell them), I listen to the prayer of everyone who calls on Me. So let them respond to Me and believe in Me in order that they may be guided aright.” (al Baqarah 2:186)

Allah(SWT) also says: "Put your trust in Allah if you are truly believers" (al Maa’iadah 5 :23)
2) Whoever does not reject the polytheist (someone who ascribes partners with Allah) or the kafir or doubts in their belief has become a Kafir.
For example if a person says that he is not sure whether a Christian, Jew or Hindu is a Kafir or not, then he obviously has doubts in Islam. Allah(swt) says in the Quran:
"Only those are the believers who have believed in Allah and His Messenger, and after ward doubt not but strive with their wealth and their lives for the cause of Allah. Those ! they are the truthful. (al Hujuraat 49:15)

Allah(swt) also says:
"Indeed there has been an excellent example for you in Ibrahim and those with him, when they said to their people: "Verily we are free from you and whatever you worship beside Allah: We have rejected you, and there has started between us and you hostility and hatred for ever until you believe in Allah alone" (al Mumtahinah 60:6)

3) Whoever wishes to go for judgement to other then Allah (Quran and Sunnah), has ascribed a partner with Allah, and has become a mushrik. This is because the right to judge, command, and legislate belong only to Allah. We must acknowledge this in theory and practice, by being judged only by Allah(SWT)'s commands and legislations (Quran and Sunnah).

Today man has exceeded his limits and attempted to take Allah(SWT)'s right of judgement, legislation and commandment. On the other hand man has ascribed a partner to Allah by acknowledging these false judges, legislators and those making commandments based on other then Allah's Sharia (Quran and Sunnah).

In order for us to worship Allah(SWT) alone we must acknowledge that only Allah has the right to legislate, command, and judge, and in turn we must adhere strictly to the commands, legislations and judgements of Allah(SWT) only (Quran And Sunnah). Allah(swt) says:
“Have you seen those (hypocrites) who claim that they believe in that which has been sent down to you, and that which was sent down before you, and they wish to go for judgement (in their disputes) to the Taghut (false judges) while they have been ordered to reject them. But shaytan (satan) wishes to lead them far astray.” (an Nisaa’ 4:60)

Also Allah(swt) says:
“But no by your Lord, they can have no faith, until they make you (Muhammad (sallallahu ‘alaihi wa sallam)) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission. “ (an Nisaa’ 4:65)
Also:
“You do not worship besides Him but only names which you have named- you and your fathers- for which Allah has sent down no authority. The Hukm (judgement, legislation, command) is for none but Allah. He has commanded that you worship none but Him; that is the (true) straight religion, but most men know not.” (Yoosuf 12:40)

Allah(swt) has also informed us by saying:
“Indeed your Lord is Allah, who created the heavens and the earth in six days, and then rose over (Istawa) the Throne. He brings the night as a cover over the day, seeking it rapidly, and (He created) the sun, the moon, the stars subjected to His Amr (command). Surely, His is the creation and Amr (commandment). Blessed is Allah, the Lord of Alamin.” (al A’raaf 7:54)

4) Whoever is displeased with anything Prophet Muhammad (sallallahu ‘alaihi wa sallam) has come with- even though he acts according to it, has commited kufr (disbelief). One such example could be a person who prays the five daily prayers but hates to perform them or a woman who wears hijab but hates it. Allah(swt) says:
“But those who disbelieve, for them is destruction and (Allah) will make their deeds vain. That is because they hate that which Allah has sent down, so He has made their deeds fruitless.” (Muhammad 47:8-9)
and:-
“Then how (will it be) when the angels take their souls at death, smiting their faces and backs? That is because they followed that which angered Allah and hated that which pleased Him. So He made their deeds fruitless.” (Muhammad 47:27-28)
and:-
“But no by your Lord, they can have no faith, until they make you (Muhammad (sallallahu ‘alaihi wa sallam)) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission.” (an Nisaa’ 4:65)

5) Whoever ridicules or makes fun of anything in the Deen (religion) or mocks any reward or punishment in Islam- has become a Kaafir. The evidence for this is the saying of Allah(swt):
“Say, was it Allah and His Messenger that you were mocking? Make no excuses; you have disbelieved after you had believed...” (at Tawbah 9:65-66)

6) Magic (Al-Sihr), All types of magic are forbidden- whether one participates, spends in it or is sympathetic towards its practice. Whoever practices magic or is pleased with magic, becomes a Kafir. For Allah(swt) has said in the Quran:
“... Sulayman did not disbelieve, but the Shayateen disbelieved, teaching men magic and such things that came down at Babylon to the two angels Harut and Marut, but neither of these two taught anyone till they had said 'We are only for trial, so believe not (by learning from us)'... And indeed they knew, that the buyers of it (magic) would have no share in the Hereafter.” (at Tawbah 9:23)

7) Whoever supports and helps the mushrik (polytheist, disbeliever) against the Muslim is a Kafir because the enemy of Allah(swt) is dearer to him than a Muslim who believes in Allah(swt). The evidence for this is the saying of Allah(swt):
“O you who believe! Take not for auliya (protectors, helpers, friends) disbelievers instead of believers. Do you wish to offer Allah a manifest proof against yourselves? (an Nisaa’ 4:144)
Also:
“O you who believe! Take not as Auliya (supporters and helpers) your fathers and your brothers if they prefer disbelief to Belief. And whoever of you does so, then he is one of the Zalimun (wrongdoers).” (at Tawbah 9: 23)

And:
“O you who believe! Take not the Jews and the Christians as Auliya' (friends, protectors, helpers) they are but Auliya of each other. And if any amongst you takes them (as auliya), then surely he is one of them. Verily Allah guides not those people who are the Zalimun (polytheists, wrong-doers and unjust).” (al Maai’dah 5:51)

8) If a person believes that he can improve upon Islam by adding things to the Shari’ah (Laws of Allah(swt)) or removing some aspects of it then he becomes a Kafir. This is because Allah(swt) sent the Message of Islam to His Prophet Muhammad (sallallahu alaihi wa sallam) for all of mankind in perfection and if anyone denies this then he contradicts the following Ayaat in the Qur'an:-

“Whoever desires a deen other than submission [Islam] never will it be accepted of him and in the hereafter he will be of the losers.” (aali I’mraan 3:85)
and
“Do they seek the judgement [hukm] of jaahileeyah? Who can be better in judgement [hukman] than Allah for a people of certainty [yooqinoon].” (al Maai’dah 5:50)
and:-
“Or do they have partners [shurakaa'u] with Allah to legislate [shara’oo] for them in the deen what Allah does not allow” (ash Shoora 42:21)
and:-
“This day, I have perfected your religion for you, completed my favour upon you, and have chosen Islam as your religion.” ( Al-Ma’idah, 5 3)

9) To love something is natural but if you love anything more then Allah, or equal to Allah(SWT) then you have made a partner with Allah and this is a form of Shirk. Allah(swt) says:
“Say if you fathers, your sons, your brothers, your wives, your kindred, the wealth that you have gained, the commerce in which you fear a decline, and the dwellings in which you delight.... are dearer to you than Allah and His Messenger, and fighting Jihad in His cause then wait until Allah brings about His Decision (torment). And Allah guides not the people who are Al-Fasiqun (the rebellious, disobedient to Allah)” (at Tawbah 9: 24)
Also:
“And of mankind are some who take (for worship) others besides Allah as rivals (to Allah). They love them as they love Allah. But those who believe, love Allah more (than anything else). If only those who do wrong could see, when they will see the torment, that all power belongs to Allah and that Allah is severe in punishment.” (al Baqarah 2:165)

10) Turning away from the Message of Muhammad (sallallahu ‘alaihi wa sallam), by not learning it or acting according to it, takes a person outside the fold of Islam. The proof for this is in the Qur'an as Allah(SWT) says :
“And who is more unjust (wrong) than one who is reminded of the signs of Allah, then he turns away from them (the signs)? Most certainly We will take revenge on the guilty ones.” (as-Sajdah 32:22)
Also:
“And who turns away from the religion of Ibrahim (Abraham) (i.e. Islamic Monotheism) except him who befools himself ? Truly, we chose him in this world and verily, in the Hearafter he will be among the righteous.” (al Baqarah 2:130)
Also:
“Say (O Muhammad (sallallahu ‘alaihi wa sallam) to mankind): “If you (really) love Allah then follow me, Allah will love you and forgive you your sins. And Allah is Oft-forgiving, Most Merciful.” (aali I’mraan 3:31)