How do I figure out where to put the fins on my board?
1. Fin placement should be relative to where your back foot is on the board... the side fins position should be right under your foot as a baseline. Do 10 pop ups on a tape measure or yard stick and measure the average distance your back foot lands from zero. This is your "neutral point'. If you want the board to be more drivey down the line position the fins back towards the tail a bit. If you want looser and more turning move them up a bit (this is a pivot point). But you can't go wrong with the typical 11"-12" up from the tail (seems to be where most feet land anyway).
2. Toe-in: More toe-in provides more turning but more drag. For front tri fin or quads a 1/4" toe in (measured by a Versa-Square or G-square) is optimal. Rear quad fins get 1/8" or 3/16" toe-in depending on if you want drive or more turning. Twins get 3/16" toe
3. Cant: Fin cant provides "lift". More cant is looser, less is drivey. This is a big decision when buying or setting the fin boxes. It all depends on how you want to surf and the wave shape you surf. 5 degree front fin cant is fast down the line while 8-9 degree front fin cant is looser and turns easier (pivot point).
Here is the fin position chart in the surfboard building guide, they are good numbers to use but don't be afraid to experiment!