Debbie, here are a couple of photos of what I did. Basically, the center fin is stock (used existing holes, but replaced the fin with the Rainbow Shapiro Surf 2.1)
I went 4 inches out from the center line (left and right) and made lines that run parallel to the center line. Then I positioned the outer fins such that the back edge of each fin is 4 inches from the back edge of the board (the board curves so it may range from 4 - 4.25 depending on where you measure). On this new side parallel line, mark the ends of the fins (I used a pencil) and then make sure the back edge is 4 inches from the back of the board.
To mark the holes to drill, I just laid the fins on their sides along the new side lines and marked the holes (2 for each fin which is the LF standard) and then checked by measuring the distances between the holes (to match the fin holes) and the distance from the holes to each end of the fin to make sure they matched on the board. The drill holes are 1/4 inch, assuming you are using the standard hardware that comes with the Rainbow fins.
I just laid the board (deck side down) on a sawhorse and drilled straight down. I carefully used a razor to clean the scrap plastic from the hole edges. The holes came out very clean and looked exactly like the factory drilled holes for the center fins.
Comment on the fins: the picture shows what appear to be 2 different colored side fins - they are exactly the same size/shape - just different colors. I got them 1/2 price from my board shop!
Hope this helps!