1. Take a long piece of 1.5" styrofoam planks, cut them into your desired size, and then tape them together. I made a 32 x 32" square, but you may want to make it bigger or smaller based on preference.
2. Take your fabric and find your selvage edge. Line it up with the edge of your square, corner to corner, and insert pins along each side. You may want to put the pins in at an angle to ensure that they go in all of the way, otherwise the fabric may travel up the pin and distort everything. Be sure to smooth out any bumps and wrinkles as you go.
3. Once your fabric is pinned into place, place your pattern piece (be mindful of the grainlines!) I am an avid supporter or over pinning your project to make sure everything is secure. Try to pin fairly close to the edges so that the fabric doesn't move.
4. After your pattern piece has been pinned, you may cut the excess fabric around the piece so that it doesn't weigh down everything and get in the way. Try to conserve as much fabric as possible!
This is personal preference, but at this point I like to take out the first set of pins (the ones along the edges of the square.)
5. Following your pattern, cut your fabric.
6. If you have any markings that need to be traced on, remove any pins that will be in your way (but leave enough to keep the fabric down), and insert your tracing paper between your fabric and your pattern.
Viola! You have a lovely, not-goofed-around-the-edges pattern piece.