Windows ect

I use an exato knife and a # 11 blade. Combine that with a metal scale rule, (mine says it's a Model Railroad Scale) and was produced by Walthers way back in the 60's when I bought mine. They are still available.

I use the scale to guide the knife and cut slightly undersize then depending on the size of the opening use emory boards, sanding sticks, home made, or a jewelers file.

As for cutting lets say a window opening. Always use a fresh blade and if your doing a lot of cutting be willing to swap to a new one. Ok back to the window, I put the scale just inside the line. Light passes with the knife and I don't go from bottom of the line to the top. I stop about a 1/4 inch from the end. You can tell when you have cut all the way thru. Then I flip the wall around and do that last 1/4 inch. Do all 4 sides the same way if you can. Sometimes the window openings or doors are just too narrow. The idea is not to have the knife cut beyond the window out line. I generally make 3-5 passes with my knife.

Turn your wall piece upside down and you can tell if your cuts meet up with each other in the corners. If not flip it back over and using the point of the blade cut into the corner very carefully. Practice on a piece of scrap and all this should start to make sense.

You can share photos here and get some clarification or more tips if you need them.

Jack

Go to a good wood supply

Go to a good wood supply store and get a  square mortise drill bit and make perfect square corners every time.

They cost  yes but they are great   

rich

Good ol' Dremel

Last year I built a Campbells kit that must have been around 40 years old. The wood was like hardwood and pretty difficult to cut, even with new Exacto #11 blades. After 30 minutes I gave up and ended up using my good ol' Dremel with their #546 cross cut blade. Went as far as I could towards the corners and finished them off with the Exacto blades. Worked like the proverbial charm!