I struggled with that as well at first. What you should do is right click on the surface you want to focus on for your 2D and choose align view. Then choose export as 2D (not sure if this is possible without the Pro version of Sketchup) and find export as PDF (from top of my head) so dimensions will stay as is (always check on dimensions in a later phase - might be one of the other file types that kept the dimensions). Then import in Gimp or a vector based program.
Might not be the exact steps, as it's from top of my head, but you should be able to manage with this I guess.