A fork is just a program based on minetest. If the creator of the fork makes a build of the fork, installing is just like installing official minetest: just download, extract and run. With other forks you will have to compile it yourself. Instructions for compiling are in the readme.txt, which is shown towards the bottom of the
github page for minetest and most forks. The
patchset page has a list of code waiting to be considered, usually in the form of a fork.
The most straightforward way to install a texture pack is to copy all of the pictures into your data folder, overwriting the original pictures. If you downloaded the windows build or are running a compiled version "in place", that should be the folder called "data" inside the folder you downloaded. That method is irreversible, so the recommended way is to add the line "texture_path=(where you put the pictures)" to the minetest.conf text file. For a windows build or a "run in place" build, minetest.conf should be right inside the folder you downloaded.