Here's how I always do it: as others have said wear no panties and a short skirt, preferably black.
Try not to sit on it when sitting down and the best place is right between fabric type couch cushions so my stream floods right down between when I lean slightly forward. It's actually better to be a little more forceful when peeing.
This way you can pretty much piss a whole bladder, even after drinking, right into their sofa without anyone knowing. I've done this multiple times in one night before at college parties.
If you create a small stain you can always "spill" some drink a