Question:
How would I use an ABA approach to teach social skills?

My Opinion:
I don't know how social your two little ones are now, or how much language they have, but from what little I can remember of your posts, they are probably more verbal than mine. But this is what I did. Hope it helps.

My two guys are very social, they just had trouble learning how to interact and how to play appropriately. For example, if they were outside with each other or their oldest brother, and someone went inside, they would cry and go inside to follow the other one, but they would not know how to tell him to come back outside. So they would cry. They were basically nonverbal, they had some receptive, the older one had a little expressive, about 50 words.

So I am teaching them how to touch their brother on the arm and say "play?" or "outside?" instead of just crying. We practice this a LOT, every time they just cry. Also, we go outside (they love outside, can you tell!) and get a ball. I taught them to say "ball" and offer it to another brother. We learn how to play games with a ball. This also works for toy cars or whatever. Right now the 3yo has no expressive, but he can come up to someone and offer the ball. The 5yo comes to me now, sometimes to his brothers. He can say "ball" or "swing" or "outside" or "water" or whatever he wants, generally 1-2 words at a time.

Also, I pick them up and throw them on the couch. This is one of their favorite games. I taught the older one to count to three this way. I would pick him up and say "one", then pretend to throw him on the couch, then "two" and pretend to throw, then a long, drawn out "THREE" and throw him. He loves it. He will now come up to me and say "one?" with a hopeful look on his face. The younger one will then come up and look up expectantly. So far, he does not verbalize tho. But eventually, hopefully.

We also play a tug-o-war game called "pull the blanket", and "run & fall down". They all like these games. I make the older boy say something appropriate like "play" or "pull" or "fall down" to start the game.

They are learning to give things to the appropriate owner. Interestingly, the younger one is better at this than the older one. He will find his brother's cup on the floor. He picks it up and gives it to him. If he tries to give it to me, I say "give it to Steven (or whoever)" and then show him how to do it. They are learning the names this way. I will give the older one both cups and say "give it to David", and help him give it. This also works for the social/play games. I say "give the BALL to DAVID" to learn nouns and names. Also to play. They are not getting these concepts very quickly and need lots of prompts, but they love it when they can play together, so the motivation is there.