Go to liquidbounce.net
Click manual download or github builds.
Go into .minecraft folder and put it in the mods folder ( if you dont already have it download forge 1.8.9 and run it.)
most scripts don't work on 1.12.2 (more known as "cross version"), due to a lot changing between versions (b72 and b73). There isn't a way to use liquidbounce 1.12.2 decently with scripts, so keep playing on 1.8.9 or use another client