I have been a serious hobby quilter for 17 years now. My 1st quilt of course was worse than my last. Worked on 365 quilt last year and really improved by leaps and bounds.
My advice matches several already listed.
1. Pay attention to what you are currently doing. Take a day with no TV, etc and concentrate on cutting and piecing.
1 day concentration compared to many days frustration?
2. Cutting: is fabric flat? ruler lined up? rotary blade sharp? Is my Pre cut really correct size? Biggie- am I paying attention to grain lines? We often don't do this is quilting.
3. Thread: I use Aurifil, but used DMC in past. 50wt. Really thin to help seams. 1/4 inch seam counts in patch (like someone already said), I pin 1 inch away from beginning, helps to not be taking a pin out immediately. Am I veering off at the end? Be consistent.
4. I finger press then measure my piecing. It is easier to fix one seam now than take out 5 later. You probably have not developed a measuring eye. Where you "see" the mistake before you measure. Also does your seam ripper "fit" your hand?
5. When I press, am I ironing or pressing? I actually made this a challenge last year and it has stuck. Also do I pull my fabric as I press? You can starch distortion into your fabric.
6. A class really helps with learning tricks not printed in books.
We all have Opinions just like we all have belly buttons.
Hope you improve but with great fun. Gi quilt