I've done them. Not as difficult as you might think. If the skins come with the edges pre-bent 90 degrees, you can "help" them over with the wide flat vise grips first to get them farther over. ease the whole edge over gradually all along the whole edge, a little bit at a time. When you use your hammer, make sure you keep a (flat) dolly on the opposite side under the edge. I have found that after removing the old skin, the edge of the inner door panel is usally rusty and should be sand blasted to clean it up first and painted. The door gets all wobbly when the old skin is removed. Take the door off and install the skin loosely! install the door, line it up, move the skin around on the inner part of the door and adjust hinges for perfect fit. THEN, tighten the skin to the door (on the car) and adjust the squareness of the door to avoid a big twist. Tack weld,check fit as you go, seam sealer, done.
I like to pre-paint the inner of the door skin first since they usually come only primed (not water proof-epoxy primer required to be water proof).
Rob
ps-I'm no expert but that's how i do it. (my dad's a retired bodyman with 40+ years experience-always had a car on the go inthe garage while i was growing up- i learned a few things).