It's just like wonder trading on X/Y. But I'll still tell you :P
Go to your PSS, and connect to the internet. Then click the little gray button on the top of the touch screen. There will be selection of things to choose from, like GTS and Battle Spot. There is a little circle on the middle of the right of the screen. It says "Wonder Trade". Click that. Then on the. Bottom screen it will ask you if you want to make a wonder trade. Click yes. Then choose any Pokemon you have in your party or your PC.
Hope I helped!
Source: Experience
