how to export .skp to .DXF with Sketchup make:
– save as / export .skp in .DAE
– save .DAE as .FBX (download fbx20133 converter, official software provided by Autocad)
– with the same method save .FBX as .DFX
Disclaimer: this is a fast guide, a tutorial, an how-to article that I personally wrote for you. Feel free to write here any kind of suggestion! It is helpful if you need to solve your problem, if you need a tip or if you are in trouble and you need an help!