on the upper left corner(where you usually put the senders address) you put the receivers address. In the middle of the envelop(where you would put the receivers address) you put your own... then you don't put a stamp. So the post office will think that you forgot to put a stamp so send it to the senders address(which is in fact the receivers real address)

If you still dont understand dont worry i suck at trying to explain these things so someone else will...