For yrs I would either use foundation papers such as Thangles or the method where you would draw a diagonal line and sew 1/4" on each side. However, I found that method quite often was not very accurate even when I would round up to the nearest whole inch. Then several months ago I decided to just cut the squares to the actual size (i.e, 2-7/8th inches), cut it in half on the diagonal, lay the 2 triangles RST and sew. Viola, accurate hsts every time! I realized that the problem before was if you do not use a very skinny line when drawing the diagonal, you will not be accurate. Plus this way is faster as you skip the step of either drawing the line or dealing with pinning the fabric to the paper. So long as you don't "manhandle" the bias edge when laying the 2 triangles together you will be just fine. You can also layer the 2 fabrics together before you cut the square, but you will probably still have to line the triangles up a bit before sewing.