Modeling interior details, like doorways and windows, is a straight-forward and easy process in SketchUp. We show you how in our SketchUp training courses.
Firstly, make sure you have moved the external walls to their own layer; then hide that layer. This will make the modeling of interior details much easier. Then, in the case of cutting out door frames,  make sure you enter into the internal walls group before any modeling takes place. Otherwise you’ll find that you’re drawing details outside the group which have no relation to the objects you are trying to manipulate.
Go to Window > Styles > and choose the Wireframe style. (Whilst X-Ray style is useful in many instances, it still tends to hide the details of the floorplan.) Now use the Tape Measure (T) and Rectangle (R) tools to create one door shape (2040 x 820mm).
Before we cut the rectangle from the walls, we want to copy it to other locations. First go to the X-Ray style. Double-click on the door with the Select tool to select it, and go back to the Wireframe style. Choose the Move (M) tool, press Control to copy (Mac: Option), and move the door to the next position on the floorplan. Repeat this procedure for all the interior doors. Some doors you’ll have to rotate 90 degrees with the Rotate tool (Q).
To cut go back to X-Ray style, and select the Push/Pull (P) tool. Click on one rectangle and push until you get the message Offset Limit or On Face. Then click and the door is cut out. The Push/Pull tool has a memory, so all you need to do for the next doors is double-click on all the rectangles to cut them out.
Many useful tips and techniques can be found at the SketchUp website. And see many examples of our own clients’ work on our Facebook page.
Other related SketchUp Blog Posts:

• SketchUp background
• SketchUp introduction
• SketchUp modeling
• SketchUp contours
• SketchUp workspace