I love First Lite stuff...it'$ pricey, I know, but I use the Solitudes for cold weather down to about 20 deg, and the Sanctuaries for cold below that temperature. I've been ok down to -25 or so (wind chill) with those. I usually put the bottoms on before I leave for the hike in, but I'm in PA, & my longest hikes are 1-3 miles, & usually are ALOT shorter than that.
Sometimes I wait until I get to the tree. But if I have a long hike in, I just stuff 'em in a dry bag & use the bottom "bedroll" straps on my pack to hold it- the dry bag keeps 'em dry, but more important, I usually hunt in really thick stuff which, in PA, means PLENTY of thorns & stickers, so they are protected & don't get caught up/torn up on the way in. Thorns just slide off of dry bags.
My bottoms ALWAYS go on when I'm on the ground. I practiced trying to put them on with a platform set about a foot of off the ground & my saddle on, and it wasn't pretty- I'm just too clumsy & off balance for that!
The nice thing about them is they have what I call "double zippers" in the legs-you have a zipper that goes down your leg, to close it up once you put your foot thru it, and then you have another one on the same track that you can open, so you can "vent" as much of your legs as you want to on the hike in & during the climb. Once I cool down a bit, I just stand up, zip the legs full closed, put the jacket on, and stay warm!