You would want to cut your squares 6 1/2 by 6 1/2 inches. I don't cut my HSTS until after I have sewn the seams 1/4 inch on both sides of the diagonal. I use an iron to press the diagonal seam on one square and then place it on top and use that to work off my 1/4 inch seam. But you could cut first and deal with the bias seam. That might mean that you pull the square out of shape though.
Once both seams are done, I cut down the middle and press to the dark side. One pressed, I "square up" using a ruler that has a 45 degree angle line and I align that to the diagonal. It helps to have a square ruler and to mark off the total size with painters tape. Just don't forget you will need to have your 1/4 inch seams on the outside.