People have three conditions:
1- A person who is capable of examining the evidence of Islamic law and have the necessary tools of knowledge of language, principles of jurisprudence, interpretation, and levels of preference (tarjih). This person should only do what the evidence and their guidance lead them to do, whether it is in their own school of law or in another. However, this is very rare among people.
2- A muqallid (one who follows a specific school of law) who does not have the ability mentioned above: This person should follow their own school of law, whether it permits or prohibits something, because permissibility and prohibition are based on specific principles. It is not suitable for them to combine different opinions from various schools of law because it would lead to contradictions with these principles, which would corrupt the methodology and the result.
3- A follower of a specific school of law but seeks a fatwa (religious ruling) from someone who is qualified to exercise ijtihad (independent reasoning). In this case, they may follow the fatwa given to them even if it contradicts their own school of law.
Fatwa by Dr. Khālid Naṣr