I recently took a class where we had to make snowball blocks. Some ladies had really wonky blocks even though they said they were very careful with the construction. We think it might have been the pressing as one lady confessed to ironing rather than pressing.
The other thing I found is that I'm not really good at sewing a straight line without a guide. I used a tool called The Angler II but you could achieve the same think with a strip of take. It has a line leading to the needed. You put the starting and ending corners on the line and as you sew, keep the ending corner moving up the line. By doing this the sewing line is straight which reduces the wonkiness.