Go into trade with another person over Xbox Live.

Player two, select what you want to give to player 1 and accept then press Xbox logo and wait.

Player 1, as soon as player two is ready accept.

Player 2 as soon as you see the screen change press yes on the Xbox dashboard button (press Y beforehand).

Player 1 you should have the traded items, but player 2 should also have kept them as well.