Placing walls exactly is important and there are many way to achieve this. We suggest you take a look at the following tutorial video. Using guidelines is the best method to start with. If you understand the basics of guidelines then fast forward to 2:03 tp see how to use them for wall placement:
Placing internal walls relative to an external wall could also be achieved using guidelines. Don't forget to use
Ctrl+W to define the walls snapping point, as you place the wall.
Placing doors and windows relative to a wall end could (or any reference point) can also be done with a guideline, but it is possibly best achieved using the
construction support tools as demonstrated in this video:
And then finally to edit existing walls, use the Wall editing tools... see the following video:
