Depending on the tissue you are intending to work on, the proportion for the protocol may differ. However, generally speaking they follow the same process and you can adapt accordingly.
Of course there is always the option to get kits ready to use and have less problems. the reason is the colorimetric assays can be affected by the sample if its slightly colorful (for example liver tissue).
The buffer can be phosphate buffer or PBS, but the most important factor is the acidic pH which is a critical factor for the enzyme activity. you may find paper suggesting pH6 or other pH5.4, I worked with both protocoles and they are both satisfactory.
This paper may be helpful for the protocol https://www.sciencedirect.com/science/article/pii/S0946672X17305217