Hi, Slench
I´d say it´s pretty much the same procedure as with any other model - learning basic SU operations, watching video tutorials, finding a decent blueprint plus lots of reference photos ( but if you´re aiming for a low poly model, you probably won´t need many of them ) etc.
This might give you a rough modeling idea http://www.dailymotion.com/video/x5riz0_uzi-sur-google-sketchup_creation - no blueprints used , but anyway...
About setting up blueprints in SU http://www.the-blueprints.com/tutorials/sketchup/
As for using your model in games like TF2, you might want to check this out http://developer.valvesoftware.com/wiki/SketchUp_Source_Tools
Hope this helps.
S