Best possible way to do LFT is to use some commercially available test kits (we use Autospan liquid gold etc.). What you require is an absorbance spectrophotometer. The protocols are written in respective manuals and very easy to perform. The kits are not only low cost but also saves a lot of time and work. For more information you can follow the publication (http://www.bose.res.in/~skpal/papers/aniruddha_fsoa1.pdf).
Commercially available diagnostic kits will do. Choose a reputed brand and biochemistry analyzer. Procedure and range of normal values will be available inside individual kits. Principles of methods used for estimation of a certain analyte also given there.
Most tests that interest you (Bilirubin, SGOT, SGPT, Alkaline phosphate) don't evaluate liver function. These are markers of various conditions, but not liver function.
Please, see this publication, for example: https://www.ueg.eu/education/latest-news/article/article/mistakes-in-liver-function-test-abnormalities-and-how-to-avoid-them/