You will need:
Foam core or cork about 24" (60 cm) square
Compass with pencil or pen
Find the point halfway across one side of the foam core. Using the ruler, mark a straight line at the halfway point all the way across the board. Do the same in the other direction. This will mark the halfway point in each direction, and will give you the exact center of the board.
Draw concentric circles on the board about 1" (3 cm) apart, centered on the center of the board. When you're finished, your board should look like this:
That's all there is to it! When you use the board, place the center of your tatted piece at the center of the board. Use the concentric circles as a guide to keep your piece perfectly round.