Fair criticism of Islam should be based upon FACTS with reference from Islamic scriptures or the life and actions of prophet PBUH and his righteous companions which are proven by authentic sources. Fair criticism should not involve mocking, insulting, humiliating or demonizing Islam or Muslims.
Fair criticism should not make conclusions based upon inferences and assumptions rather they should be made on the basis of facts with specific reference from the Islamic sources.
Fair criticism of Islam should be based upon the interpretation of the Islamic scriptures and history as per the explanation of prophet PBUH and understanding of pious predecessors & scholars of Islam and everyone is bound by this interpretation if he speaks about Islamic religion.
For example, Muslims are not bound by Islamophobic interpretations of Islamic scriptures or history however everyone (who want to learn, follow or criticize Islam) is bound by Interpretation of Islamic scriptures as explained by prophet PBUH and as understood by the pious predecessors and the scholars who followed them.
To criticize anything you need to know it well, hence first of all a person need to check his source of knowledge about Islam. To gain knowledge of Islam he needs to read the Quran, Hadith and the Seerah (Biography) of the prophet PBUH and his companions R.A. Also, he needs to learn from Islamic scholars past and present and authentic books written by these scholars at the bare minimum.
Sadly most people start criticism after reading few misleading articles about Islam on Islamophobic websites or listening to debates, lectures etc from islamophobes themselves.
Lastly, people need to differentiate between Muslims and Islam and realize that if someone has Muslim name doesn’t mean that all his actions are inline with Islamic teachings. If a person chants Allahu Akbar doesn’t mean that whatever he does after that will be Islamic. If a Muslim has a beard doesn’t mean he is a devout follower of Islam.