Ummmm, that depends.
On the wrap (how wide is it?), on the baby/child being carried, and most of all on the carry.
For front carries, I don't find seats this big necessary production. I bunch the horizontal pass, sit child on it with legs in an "M", their butt hanging over, and pull the top rail up their back. Hey presto, magical "seat" that doesn't need any yanking or tucking or faffing about. I think of this like sitting in a hammock crossways - the rail is taut and I can dangle my legs freely from the knee,but I'm supported through my thighs and spine. How does this translate to a back carry?
I'm anti-pants tucking. I feel like it messes with the tension of the entire wrap, and when are we likely to use this? With smaller babies who actually need really nice snug wrap jobs.
Santa toss (pre-cocooning a small baby) places the wrap around baby the way we'd like it to sit when they're
transferred to our back. Snugly cradled, with the wrap smoothly hugging their torso and the excess bunched loosely under their knees and bottom- this is going to form the "seat" as they're rolled over a shoulder and gently moved down our spine into a high, tight ruck.
Older babies with good head control or who are sitting independently might be supermanned or hip scooted with the top rail of the wrap already held around their torso or shoulders. If you have a wiggler, it's great to secure tails under your foot, between knees, or (if you really have to) under a chin. Keep your back straight and shoulders back! Rolling your back/shoulders will give you all kinds of weird slack ;)
I like to hold the tension in the top rails, reach back under baby's legs and pull the wrap down smooth over baby's back, then tuck all that excess up a little to make that hammock rail. The legs usually spread easily into an "M" and you can easily grab up all the slack in that bottom rail and bring it up to your shoulders, where you can tighten strand by strand and sandwich the bottom rail under the top if you like. Don't forget to stand up straight (with a hand checking baby's seat) to let baby's bottom sink a little and ensure you have your shoulders back!
If you can't reach your baby's butt (common issue!) don't even try. Just sandwich. Bringing the bottom rail up onto your shoulder and gently tightening will create that "sack" shape in your wrap and make space for baby's bottom to sink - so when you stand up straight with one hand holding both tails at your throat and the other on baby's bottom, you'll feel them sink a little. Tie off and gently push upwards on the soles of baby's feet to encourage the knees to bend and seat to deepen.