Sometime it’s very hard to cut off some panels even with sharp knife. Some models, especially tanks have rather thick plastic parts. To cut it easier you can use hobby drill to improve cutting.
Drill some holes around panels you want to cut off.
As close as possible but not to damage nearby plastic.
Now carefully cut off excess of plastic with hobby knife. And it’s done.
Now you can fill holes with your own ideas :)
Below is an example how I use it on Nurgle Razorback.
Hope you like it! ;)