I learned how to do it from this tutorial:
http://www.calclassic.com/sketchup_tutorial.htm
Basically this is what you need:
1. SDK's installed and updated (including the pre-requisites MS C runtime library, the .net stuff - there is a forum here on how to install the SDK)
2. Makes your model in Skecthup then export as a collada (.dae) file
3. Import into ModelConverterX, fix textures and rename texture files, save as a .mdl file
4. Make a new library or open existing one in LibraryCreatorXML and save
5. Use Airport Design Editor or your favorite tool to get the object from the library and place it in your scenery file
6. Put the scenery file in an active Add On Scenery scenery folder and the textures in an active Add On Scenery texture folder
7. Start Flight Sim and fly
The tutorial linked above helped me learn the steps and then I made a checklist for myself to follow.