I have followed a good blender help doc 2.5 and I can join meshes together and change the dimensions height with mouse. These help docs in 2.5 are not easy to find.
Any I have a mesh of a house and I wan to cut a door.
Am I using the knife tool or what method should I/can be using.
It depends on the house mesh . post a blend file or a picture or the house in edit mode. Also it depend on what results you want. Knife cut is fine. Knife cut mid point and move edge on axis to keep straight .
What I found was subdivide the cube and remove vertices/faces. this worked well for now.
Booleans, but you’ll have to clean your mesh up a bit afterwards.