1) Buy a perfectly good fin...okay...or not...you're gonna want to start with something kinda trashed..fins aren't cheap, this isn't news to any of us...
Then template your new fin onto it:
2) Cut that sucker out with something really sharp...pref. a carbide bladed-jigsaw or band saw if you happen to work with carbide tipped things a lot...i just use my grinder at different angles, it's easier to 100% true up the template after it's rough foiled. So get it really close.
Then Snag your grinder and get itchy. Here is my fin rough sanded with a 60 grit disc. Make special note of your foil lines, wobbly ones suck. they're everywhere, I can't say it'll noticeable affect 90% of surfer's experiences, but it shows a little bit of a lack of quality to me...okay, rant over...here's the rough foil:
3) Then get funky with a soft pad and some 120 sand paper, take your grinder scratches out and smooth off the foil. repeat with 220 then i finish mine off with 320 wet/dry.......Then paint something on it, so as to advertise to the world that yes, your fin is cool:
5) The soon thereafter, shred your sled on something resembling: