First post - sorry if it is all screwed up. Had to learn how to post pics.
This is the method I use to make wheel wells. There are obviously many ways to do this but this works for me.
1. I use my old decks to landmark where my trucks contact the deck and mark it on the new board as the center of the wheel well as best I can. I make 2 more lines on either side of that to use as boundaries I go about 2" wide at the edge of the board.
2. I use a piece of 4" PVC pipe (or whatever you can find in a somewhat similar diameter) and glue a piece of sandpaper to it (I use 60 or 80 grit).
3. I carefully saw in back and forth motion on an angle using the centre line I drew in step 1 to guide me for the angle and the boundary lines to tell me when to stop. I usually go until I hit the 6th layer of wood and then stop there.
4. I smooth up the wells with finer sandpaper.
5. I use some minwax clear wood finish or whatever is available to seal the spots where I sanded.
This is tedious work and not something I would want to do on many decks at a time, but for me I don't mind doing it.