gearbox issue

Hey guys, new to the form trying to get into hobby. Recently purchased a fully disassembled and rust hunk of junk 125cc Chinese pitbike and over the last few months put it together no issues, with pretty decent success. having had that turn out i fingered i would get another project but...

Is there anyone here that can explain how to shim a box, where to shim and shimming the clutch basket in easy terms to understand. I carnt seem to find much info on this seems like some people know a bit on this but don't put it up on the web. I'm just putting a new gear set in my yx160 and I...