A few tips for block construction...
Starch is your best friend. At this point, if you haven't been starching, it is very beneficial. It will help your blocks keep their shape better when putting the quilt together. I prefer to starch fairly heavy during this step, then when it comes time to put the quilt together I will only use a spritz of water. You can find my starch recipe here.
Square up as you go. Make sure your individual units are squared to size. Square each step. This will help prevent wonky blocks.
Pin, pin, pin. I don't like to. Plain and simple. BUT, when putting together this block, it makes it so much more simple. It prevents time with a seam ripper. Trust me, I know. Even if you only pin the Triangle-in-a-Square and Square-in-a Square points, you'll be good. It really does make a difference. I typically (and prefer) just nest seams, but it is very easy for the points to be off.