The blank is 1.25 inches thick. If you do not tension your blade correctly is will warp during the cut. This will distort the ornament and likely break it. Get the tension good and tight. Make sure you let the blade do the cutting. Don't force it. Don't apply too much side pressure on the blade while you cut. You want the blade to be able to cut straight up and down.
Use a less dense wood if possible. The cut will be easier and help reduce blade warping.
Make sure you have you table exactly 90 degrees to the blade. If the ornament will come out one way but not the other then the cut was not straight.
The compound cut clamp you see in the tutorial is available in my catalog under the "Tools" category.
Click now and make a family heirloom this weekend.
DVD and the 2013 update DVD at this link. DVD #1 is $20 and contains over 1,100 patterns. The 2013 update DVD is $7.50 and has another 178 patterns. The 2014 update DVD is $7.50 and has another 223 patterns.