any number of decks
Step 1: Design
Where theta is 360/number of pieces
If you're decks (or any kind of board) are not 8" wide, you can change the 8 in the formula for x to the desired width.
You have to keep in mind that you can only cut about 12 inches of the middle of the skateboard (within the wheelbase), after that the contour changes and pieces wont fit together uniformly when you put them together. You'll also run into the holes for the trucks.
So, if you're not using the spreadsheet and you want to do it by hand, you have to find out how many pieces you can get out of one skateboard. if you're using 1 piece the total width will be "a" if you're cutting 2 it will be "a+(b+x)" if you're cutting 3 it will be "a+2(b+x)". The pattern is L=a+(n-1)(b+x), where n is the number of pieces.
Step 2: Cut
On the topic of precision, mitre saws are not. If I had to guess they're probably accurate to within +- 1 or 2 degrees. If you're cutting a lot of pieces at small angles (like I was) your wedges are not going to be identical. This means that at the end you will have to compensate, probably by adding an extra piece, which is what I did. If you have some extra wood you could make practice cuts and find where on the saw the angle you're looking for is.
Keep in mind that that the angle "theta" mentioned in step 1 is the angle the piece will occupy within the ring. Another way to think of it is the angle the trapezoid would make if its sloped sides met and it formed an Isosceles triangle. The "mitre angle" is the angle you will be cutting at, which is half of "theta"
Step 3: Glue
Step 4: Drill
Step 5: Sew
There are probably many ways to do this. I cut a very long piece of twine, about three times the circumference of the ring. On each end I tied a nail. String it through the first hole. Bring the ends together and put half the length on each side of the hole. Sew all the way around with one length and all the way around with the other (same direction).