The other trick is to make your half-square triangles just a little too big, then trim them down so they're all the perfect size.
Let's get sewing:
1. Start with a square that is 1 inch bigger than you want your final half-square triangles to be. For example, I wanted 3" squares for this pillow (3.5" before piecing), so I cut 4" squares.
2. Draw a diagonal line corner to corner on half the squares. It usually shows up better on lighter fabrics.
3. Match two squares, right sides together. Sew 1/4" away along both sides of the diagonal line that you marked. Don't sew on the diagonal line!
4. Cut on the marked diagonal line.
5. Press seams toward the darker fabric.
|side note--I really need to make a new ironing board cover, this one is looking kinda ugly|
6. Use a small square ruler and your rotary cutter to square up and trim. Align the diagonal line on the ruler with your triangle. Here, I want my square to be 3 1/2". I trimmed off a little on the top and side. Then, rotate and repeat on the other two sides, and you get...
...perfect half-square triangles, just begging to be pieced into something beautiful!