Hammer and dolly gives the best finish.
The skinning tool is really only used for final tightening of the fold.
If you try and use a door skinning tool before fully folding the skin, it will leave you with kinks and ripples like Neil said.
So either way, you will need to fold it with a hammer and dolly first and if you do it nicely, you will not even need the skinning tool.
Door skinning requires a lot of patience. Each run of the fold (bottom and sides) should be done in stages.
You want to hold your dolly firmly to the outside of the skin and gently start hammering the skin over the frame.
For example, start one side of the bottom and start hammering the skin over about a third of the way, do this all the way along. Then start again hammering it a further third and then again the rest of the way.
Keep the flat of your hammer nice and square or you will end up with blow marks on the folded edge.
I usually run a body file over the outside of the skin after folding is finished along the bottom and up the sides to even out any slight imperfections but this stage isn't really necessary unless you have a file.