'pull up your pants' generally means the pants are around your ankles or not completely on. 'Pull up your pants' means to complete the action so that the pants are fully on. 'pull on your pants' generally means the pants aren't on at all, not even partially. 'Pull on your pants' means get your pants and put them fully on. BUT, sometimes this expression will even be used if pants are already partially on. 'Pull up your pants' also can be an idiomatic expression saying, 'Don't be slack/lazy, get yourself going/motivated'.

Tighten your pants better. colloquially, you can say stretch your pants, but not in writing.