Here are some tips that might help you.
The more accurate you can be on your HSTs the easier it is to sew them together.
On the first block - on the rt side if you sew along the last 2" of the middle seam to that outside edge you can tighten that seam and pull that edge into a straigher line. Sew over from the seam about an 1/16" or a 1/8" depending on how much it takes to make it straight.
On the left side I would pick out the last 2" of that middle seam and tack or pin the corners of those two blocks together. Then sew them back together and you should have a straigher side. then trim the block square leaving your 1/4" seam allowance.
My easy to make HSTs and tips
http://www.quiltingboard.com/t-37915-1.htm
http://www.quiltingboard.com/t-23252-1.htm