For the processing I use a caliper, a steel row, snap-off blades and a square.
A brief explanation of my modeling experiences with Forex. As a base I use 2 mm thick Forex, a PVC foam sheet. And voor the gluing I use Ruderer L530.
First, the recesses of the windows and doors are cut in the top material sush as PVC stone plate or 0,5 mm styrene.
Now the windows and doors can be placed and fixed with a dot of wood glue.
Finally, the facade can be glued to the floor with Rurderer L530. I do use a piece of masking tape or fix it.
Keep checking that everything is perpendicular and straight. After all, nothing is more annoying than that your building is crooked.