I would prepare myself to seat my DS by putting my right hand under his bum, forearm between his legs, and my left hand across this back and on his far shoulder. I would lower his bottom onto the seat and, as I was pulling it out, grab the crotch buckle.
As I was doing this, I was ALSO laying his back onto the back of the seat, and as soon as that was done, lay my left forearm from elbow to wrist from his hip up to his shoulder (elbow at hip, wrist at shoulder), then slide that "male part" quickly into the crotch buckle. Side 1 done.
Keeping my left forearm against his abdomen, grab other shoulder strap and "male part" and quickly put it in. Side 2 done.
Then, with the beautiful Evenflo Triumph side knobs, tighten and go!
Whenever I took him out, I pulled the harness straps way loose (again, loved those side adjust knobs) so that there would be no struggling with getting his arms through when it was time to put him back in. hth. Fun stage, right?