I'm sure I'm missing the obvious...

Ive managed to list all the ones we'd like to swap, but can't work out how to add the ones we are looking for . Can someone give me idiot proof instructions? I'm on a phone 

Click the open button on the top left, then click add requests, then click on the set you're looking to add cards from and then you can use the drop down menu of the cards to pick condition, and style. Hope this helps.