First, cut squares of your two fabrics 1" larger than the desired finished size. For example, to make HST of finished size 4-1/2", cut squares that measure 5-1/2" x 5-1/2". Next, on the back of one of the squares draw a diagonal line for one corner to another. Layer your two squares, right sides together, and stitch a 1/4" away from both sides of the line. Now, carefully cut along the drawn line separating the two HSTs. Set your seams and press the seams toward the darker fabric.
The HSTs you've made will be slightly larger than our desired size - 5" in our example (4-1/2" + 1/2" for seam allowances). Using a ruler that is equal to or larger than our desired size and that has a 45 degree line marked, align the 45 degree line with your seam line. The HST should extend slightly past the 5" markings and the edges of the ruler. Trim two sides. Finally, rotate the HST 180 degrees and align the 5" markings with the edges of the unit and the 45 degree line with the seam. Trim the remaining two sides.
Repeat the trimming on the second HST. That easily you have made two accurate HST units without handling any bias edges. That are many more ways to make HSTs so I'll be back with more.