Solosis and Rufflet (the latter caught at Lv.48 at Village Bridge - to speed the leveling up to evolve - prior to the trading) have already been traded to me and fully evolved. I missed the in-game NPC and did a Cottonee/Petilil trade as well., using my Stone on that before noticing the NPC while reviewing the Nacrene City article on Bulbapedia. LMAO
When trading on Wireless, you get to see what your trading partner is doing in their boxes; in other words, both traders can see the Pokémon being selected. It always starts where your party is located, and Box 1 is to the right, while Box 24 (the highest allowed number of boxes, which if you're curious can be obtained by first having a minimum of 1 Pokémon in the initial eight boxes at once, then doing the same with the new total of 16) is located to the left. I renamed Box 24 "TRADES" (you would not see this nor can I see your names, I think). This means anything I plan to trade at some point is in there (right now, I have a Swellow in there, and possibly a couple I have not relocated yet).
Typically you can select up to three Pokémon; this would be like saying you can pick any of the three for any of his three. Since trades can't be done three at a time, there's no point to choosing more than one most of the time (which is why I like listing it out, then it's just following the list; I almost got thrown for a loop in my last session when a Landorus was going to be traded and I'm rushing to PC on my phone saying "No, not Landorus" LMAO). Once your tradeables are selected, the screens change sides and you get to pick one (again, usually the only one). Trade is confirmed by both parties following the selection, as a final "Are you sure" type of thing, and once both confirm, you see the graphic of your Pokémon being Pokéballed (even an Egg), a graphic of the two trading places outside the balls, and the Ball then bounces in front of you producing your new Pokémon. It then says it's stopping communication (I got confused by this at first, but I believe the game also saves here), and the trade window pops back up to do it all over again.
On any screen, pushing B can cancel trading. On the main trading screen, if nothing is selected, pushing B ends the session. The exception is after the final confirmation, when the trading process begins; you can no longer cancel at this point.
The room used for Wifi trading, battling, etc. is the middle room on the upper floor of the Pokémon Center. It looks a lot like the Union Room to the left of it, it's just the wireless version whereas the Union Room is for people in the same area.