Okay, so you know how to address your envelope to be sent via IGM, right? What next? Stamps! (If you don't know how or need a refresher, you can go here.)
Well, this is pretty simple: It works like everywhere else! The difference is you don't need to put the postage anywhere specific, because different planets have different methods of mailing things.
On Earth, we put postage in the top right-hand corner, so I usually put my stamps somewhere to the left of the to-address and below the from-address, so that we can keep it separate. Of course, some people are very messy, but on Earth they will sometimes send things back, and I don't want to risk it. Plus, why not give them a break, huh?
As I said in the previous post, those stamps are cancelled out before getting to Earth. You can also cancel your own stamps, like you can have your own USPS cancellation. -- This requires your own IGM number and cancellation stamp (mine).