Up to jump, Left & Right to move, Down to fall faster, and T to open chat menu. Grab opponents flags, bring it back to your base. Tag opponents if they are on your side. If you are holding the flag then you will walk a little slower so that the other person actually has a chance to catch up with you.
This is still a W.I.P. You can go on and wait for hours to see if anyone joins, or you can tell your friends when you are on and play with them! (If they have a Scratch account and are able to use cloud data)