This instructable goes over how to make a round frame from used skateboard decks. For more information you can visit my blog.
any number of decks
Step 1: Design
The first problem is how to make the largest ring possible with the materials that you have, as well as find the angle you will be cutting at. An optimization problem, essentially. To solve you can use the set of formulas below that calculate the length of each side of the trapezoids that make up the ring relative to the number of pieces being used and the radius of the ring. If trigonometry isn't your bag, you can download a spreadsheet from my blog that will do this for you based on your inputs (excel is required).
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
Pretty simple. Just make your cuts. if you don't have an electric mitre saw you can use a mitre box and hand saw. Or if you have a lot of patience, draw a line and cut with a hand saw. Or, if you're Chuck Norris, use precision round house kicks.
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
Use wood glue to hold the wedges in place. Let it sit for a day for good measure.
Step 4: Drill
Once the glue has dried you should be able to handle it. WITH CARE. Wood glue is strong, but it is by no means ready for a game of skateboard ring Frisbee. Put it on top of some scrap wood or on a floor that is impermeable to drills...Drill four holes in each piece. One at each corner. You could measure these, but I didn't want to mark 100 holes so I just eyeballed it.
Step 5: Sew
This isn't your grandmother's sewing, this is sewing with NAILS and TWINE!
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).
Step 6: Centerpiece
That's it! what you choose to put in the middle is up to you. I don't have any pictures of the mirror being attached, but it wasn't anything special. Since there are holes all the way around the ring I tied the mirror to the back using more twine. I did this by making an X on the back and tying knots on the ends to keep it there. There's probably a much safer way to do this, but this worked for me.