Here's a tip I use when piecing angled pieces. Put a pin through the seam 1/4" in from the edge. Then poke the pin through the seam 1/4" from the edge of the other unit. Hold the pin straight up and down between two of your fingers (don't come back up through the fabric with your pin, just hold it in place) and take another pin and pin on each side of your first pin, then sew your seam, making sure to hit the "spot" thats marked with the first pin, removing the pins as you come to them. Kinda hard to explain! When you put your pin through the two seams, you'll see why it's so hard to sew them correctly because the seams don't line up.