Slow down! Take your time with all the steps: cutting, sewing and pressing.
I think my biggest leap came when I realized that a 'quarter inch seam' was a moving target. first of all, you don't measure the seam, you measure the patch. Here's how to check:
http://www.quiltingboard.com/tutoria...ce-t89997.html
And the second thing is that different fabrics and thread weights can affect this. In other words, you need to do it at the start of every project and adjust it if needed.
Obviously, the seam allowance isn't going to matter if your patches aren't cut accurately. Use the same ruler or same brand of ruler for your whole project & consistently place your fabric the same way each time. Right on the line, or just to the left.
And finally, pay attention to your pressing. Make sure that you aren't pressing a tiny fold in the seam, and pay attention to any pressing instructions in your pattern. It will help the block assembly.