Ok, the easiest way to do it as far as i know is to get SAMI. it automaticly installs mods for you really fast, all you have to do is download and save the file, then SAMI can run you thru the rest easy. I think SAMI is downloadable in the GTA San Andreas sub section of tools, if not that try mods or other ones. Before I got SAMI the sub didnt work at all, it was just a dinghy that was submerged and could only move down and it didnt work at all.
P.S. even tho the submarian sounds cool it really isnt, there are some probems with it, one is that you cant use it in the Boat School because its not fast enough and goes under the checkpoints, another is that it is sorta slow and hard to turn, but it is good if your trying to get things under water because you can go underwater, push Reutrn, and even tho your underwater you can breath there and restore your breath, thats probebly the best feature.