(This assumes you are on the left side of the turtled opponent.)
Opponent is down in a turtle position, you are based out next to him (left foot out wide for support). You reach down with your right hand on the right side of his neck (reach down close to neck, not a wide grab, like you are punching at the ground). Wrap right hand around neck and grab opposite lapel (the back of the opponents neck is in your armpit, your weight is pushing his neck down). Take right foot and step out past opponents head. At this point two things could happen: 1) If your grip is tight around his head, a collar choke is taking place (should tap) 2) If you don’t have a tight grip, you can take your left foot and then step again over your right, essentially wrapping your arm more around the neck for a choke.