One of my dive buddies would sit on the ground, strap into his harness, cross his legs, lean forward slightly, and then stand up! In manifolded PST LP 104's!! He wasn't particularly athletic (didn't have an athletic build). It always amazed me to watch him do this.
My approach, with my manifolded HP 102's, HP 120's, and (now) LP 50's was/is to sit on the ground, strap tightly into my harness, lean forward so that my weight and the weight of my doubles are on my knees and hands, swing my lower legs under me, and then stand up. Slowly. Helps if my hands can push down on something a bit higher than my legs, but this is not required. I was/am always prepared to roll to the side if things get unsteady. (I am not about to risk my back--especially now, since it is nearly 70 years old!)
ETA: We purchased a minivan (Honda Odyssey) when we learned that our first daughter was going to have twin sisters! My girls are almost adult now (ages 22 and 20), so I really don't need a minivan anymore--except that it makes this kind of thing (transporting, setting up, donning, and doffing doubles) so much easier!
rx7diver