Just enter both sides of the trade they way you want them, then click Offer Trade. The diplomacy window will vanish and it'll feel like nothing happens. DON'T REOPEN THE WINDOW.
When your friend logs in, he'll get the screen, see you accepted, and be able to click, Offer Trade. The window will vanish, but the deal will have gone through. If he closes the window wihtout accepting your offer, it will vanish and he'll have to start over.
If you open the trade window after sending him an offer, it'll erase your offer and you'll have to do it again.
It's best to work out the terms via email or something because of how touchy the diplo screen is.