Thill slip bobbers have a VERY small diameter hole through them. If you choose to use one of the "bobber stops" that you can buy you have to install that on your fishing line first. Then put your fishing line through the bobber from top to bottom. You slide the string type bobber stopper up or down the lne to adjust depth. The little red plastic ones that you thread on also slide up and down the line, but have to be loosened up first. I never messed with those kinds of bobber stoppers. I just use a small piece of rubber band....the smaller the better. Tie it tightly to your line at the desired depth and snip off the tail ends so it goes easily through your line guides.. You can slide the rubber band on your line to change depth. Good luck.
OH... and if you use minnows, I suggest hooking them through the lips/head because perch almost always take minnows head first.