I attended the NMRA Mid-East Region convention last year and learned a really great tip for a common problem: cutting square holes for windows or doors into styrene sheets.
The technique came from Tony Segro’s “Styrene Structure Scratch Building” clinic.
The first step is transferring or marking your target hole on the sheet you’re trying to cut into. This is usually done from a drawing. Next, scribe lines around where the hole must be using a sharp knife. Follow those up with a scribed “X” from the corners, breaking the hole to be cut into a set of four triangles. Lastly, a hole can be drilled in the center.
Now the triangles can be easily snapped out leaving the nice clean edges where you need them.
The tip is summed up in this image I took of his example: