To cut the rough size out, a straight edge and utility knife works well. Score it deep and then bend it back a time or two till it snaps.

After molding I sometimes cut it close to final size with offset tin snips. I just do one thickness at a time with the snips not through both sides at once. This helps to reduce sanding.

For final finishing I use a belt sander, and or dremel with the little sanding drums.

I am far from a pro at this and still learning myself.