Go to the one player menu, then it's probably under towers > challenge towers. I forget now, but it's easy enough to find. Basically, you do a tower which involves fighting opponents and doing various in-game challenges, and then you send me the tower and I have to match or beat your score. It's pretty simple to work out. So far I've won one and lost one. You don't have to send challenges to anyone (you can just repeat new towers yourself) but I think you get an achievement for doing so.
See how you get on anyway. It's basically the same as doing a normal klassic tower, but with challenges like not jumping during a match, or getting X number of sweep attacks.