The general method is to grind the material in the presence of an aqueous pH buffer and whatever other ingredients you consider necessary to the purpose. The method of grinding depends on the material. Consult the literature to see what others have done before you.

After the grinding step, the extract is centrifuged to pellet the undissolved solids. The supernatant can then be used to measure the protein content.

Dear Adam B Shapiro, Thank you very much for your answer.