Half-square triangles are my nemesis. I think I've tried just about every method for making them, and still, I sometimes get mixed results. The key I think is being careful and taking your time, something I don't always want to do when I'm trying to get a quilt done. The other key is choosing a method that'll work best with the quilt you're trying to make.
Here's a handy-dandy method that makes two matching HSTs. Use this method when making blocks that require pairs of lookalike HSTs or scrappy quilts that require a lot of HSTs, like Strawberry Preserves, a quilt from my book Idiot’s Guides: Quilting.
1. Figure out the size of the HSTs you need
For example, take a look at this block. It measures 8” finished (8.5” unfinished). The four HSTs in the corners are 2” finished (2.5” unfinished).
2. Cut two squares of fabric
To figure out the size of the squares to cut, add 1 inch to the finished size of the HSTs in your block. The HSTs in our block are 2” finished, so you need to cut two 3" squares, one pink and one brown.
3. Draw a diagonal line from corner to corner on the back of the lightest square
After drawing the diagonal line, draw two sewing lines on either side of this diagonal line, 1/4-inch away. Marking the sewing lines makes it easier to be precise, especially if you’re a beginner.
A quarter-inch seam marker like the blue one shown here is a must-have for this technique--it really makes the process of marking those lines easier! You can buy them in several sizes, so consider purchasing a set so you can use them regardless of how large the HSTs are in the quilt you’re making.
4. Place the squares right sides together, pin, and sew on the two sewing lines
5. Cut the squares apart on the diagonal line
You don’t have to cut exactly in the middle—just between the two sewing lines.
6. Press toward the dark triangle
7. Trim the HSTs to their unfinished size
The HSTs in our block are 2” finished, 2.5” unfinished. So you must trim your HSTs to 2.5 inches.
Place the 45-degree mark on your ruler on the diagonal seam, and trim the right side, then the top side. (If you’re left-handed, trim the left side, then the top.) Notice that I’m not trimming the HST to exactly 2.5” just yet; I’m leaving a bit of wiggle room for squaring it up when I trim the other two sides.
Be very careful when trimming the top so that you don’t wiggle the ruler or slip the rotary cutter. A turntable cutting mat is handy for this kind of trimming, although I find that mine wiggles a bit when I press down to cut.
If you’re new to this rotary cutter business, you might find it helpful to wear a rotary cutting glove on your non-rotary cutting hand to protect it. Even though I’m no longer new to rotary cutting, I sometimes use my glove whenever I’m cutting across or towards myself, just to be safe.
8. Rotate the HST, and then trim the other two sides
Place the 45-degree line on the seam of your HST again, and trim the remaining two sides. Now’s the time to trim the HST to exactly 2.5”.
9. Repeat the trimming process on the other HST
You now have two matching half-square triangles, ready for piecing into your block. Of course, our block requires four HSTs, so we’ll start over with two new squares and make the two additional HSTs we need.