Well, that isn't really the idea behind it. What you do is:
1. Go to the GTS and browse for an easy trade you can complete in a few minutes.
2. Go catch the pokemon required to complete the trade.
3. Come back to the GTS, put the pokemon you wish to evolve in as an offer and make sure it is holding whatever it may need to evolve.
4. Make the trade you searched for on the GTS; as in, search for it again.
5. Then, hope your pokemon you just put in hasn't been traded after those few seconds it took to make the trade, pull it out, and voila, it's evolved.
The reason why this works is because you trick the game into thinking you made the trade by putting the pokemon in the GTS and leaving it in there while you trade something else. I'm not sure if this is a mistake in the game's programming, or if it was intended to facilitate trade evolution.