I’m trying to learn a thumb grind, but my DM has the plastic siding it comes with. I tried using a knife to pop it out, but I’m just scratching the plastic. is there a trick to getting it out?
Yup! If you get a suction cup, and just push it on, then pull it off, the cap will come right out 
thanks! Now where to find a suctioin cup is another matter!!
You can also use duct tape. Just stick it onto the caps and pull it out. Alternatively, you could actually practice thumb grinding with the caps on. This can help to improve your control of the yoyo when thumb grinding.
thats what i want to know too.