There are a couple of ways to help with hst's.
One is a pattern that you can sew along like thangles but here's a free link to print out:
http://www.blockcentral.com/a-halfsq...glepaper.shtml
The 2nd way is first to get a 220 grit piece of sandpaper and duct tape it around a small piece of cardboard. This will hold your block for you to make a line from cornor to cornor to draw your lines easily.
3rd you can make your blocks just a tad bit bigger and then square up to correct size.
Good Luck!!