no need to remove the trim, i installed leather in my truck a few months back and i did exactly what your wanting to do.
To remove the bottom cusion there are simply four nuts you remove from under the seat.
Look in front under the seat and youll see them at the farther corners. two up front two out back, and the bottom cushion simply lifts up. be carefull cause there are a few wiring pigtails you have to disconnect before you pull it off. Slowly pull it up and youll see them.
Remove the straps holding the sides down from the leather and fold them over to reveal the foam (or lack thereof) and seat pan.
I simply added an extra layer from some arts and crafts none degrading foam from wal-mart, and strapped the leather over and its doing great.
The seat pan was pretty rough on mine, and thats what i think caused the ripping of the foam as i got in and out the truck, so i took my die grinder and ground that area of the metal in a round smooth fashion and added a little door lip trim (cheapo stuff from auto parts place) to the edge so it wouldnt cut into the foam anymore.