Serenity utilizes startup bytecode injection to make small edits to the game, layering its cheats over the top.
This means that, unlike other clients, the version of Serenity used is independent from the version of mods used alongside the client. Any Minecraft 1.8 Forge or LiteLoader mods will work with the client.
Installing the client is as easy as running an installer JAR. (or alternatively, fiddling with some JSON.) Pick your OptiFine (or LiteLoader) installation, and just hit the 'Install' button!
Modules in Serenity are backed by a value system allowing you to tweak and change how modules act. If the defaults don't satisfy you, or the server you're playing on has relaxed anticheat measures, make the client as extreme as you want it to be.