Numerous papers are published with protocols described in details, just google.

Dear Somaddar,

In plant extracts, phenolic content was contributed by a lot of compounds. This is the reason why you can not determine exactly the composition of phenolic compounds in your extracts whether you regconised what main substances in your extracts and you have the standard compounds for quantification. Instead, in organic chemistry, we can evaluate the content of total phenolic compounds by photometric analysis (UV-VIS) based on gallic acid. You can read the document below or use the ISO protocol to do this. Good luck to you