I agree with Farm Quilter, you need to trim all 4 sides of the HSTs. Get a square ruler and place the 45 degree line on the seam. Trim 2 sides square removing the least amount of fabric you can. Then rotate the ruler 180, place the 45 degree line on the seam and the already trimmed sides on the ruler marking for the size block you need. Trim the final 2 sides.
I would go to youtube, put in "square up quilt" in the search box and view the videos.
Also, you might want to take the time to repress the pieces using a heavy dose of spray starch. Remember, PRESS, not iron! Do this before you trim.