If you botch something, you make a mess of it or you ruin it. If you totally botch your lines in the school play, you stammer and stutter your way through the whole thing.
Interestingly, the word botch originally meant the opposite of what it means today. The Middle English word bocchen meant to mend or repair. As a noun botch means an embarrassing mistake or something that is done poorly, especially due to lack of skill. If they've never painted before, your friends working on set design might make a complete botch of the scenery for the play, which might involve repainting the whole thing.