If you want 2 half-square triangles from 2 squares, you add 7/8" to the finished measurement.

If you wish to end up with 6" finished units, cut both fabrics at 6 7/8", draw the diagonal, and sew on both sides 1/4" from the line. Then cut on the drawn line and press to the dark side.

Many people would cut these at 7", and then trim the half-square triangles after sewing, but I find if I cut, layer, and sew carefully to begin with, they are 99% accurate.

Thanks Mary. (I'm going to piece the original square, so I don't want it to be too big, so I'll go with the 7/8 rather than adding an entire inch.)