Try looking at this thread:
Forums at Splitcoaststampers
Essentially, take whatever size is the width of your frame and measure in on one side of the longer side that size: e.g. if the width is 1/2", measure in 1/2" on the long side and draw a line from that point in a diagonal to the opposite corner, making a triangle. Cut along that line (see attached).
Make sure you match up your design on your paper; for example woodgrain: if you have the grain going up and down on the longer sides, you'd want it to do the same on the top and bottom pieces. That's the nice thing about doing frames, you can choose to match the direction of the design on the paper OR have the longer sides match each other and the top going one way and the bottom going the opposite...
Good luck!
Joan