When you build something with a lot of angles like a picture frame, precision matters.
If one angle is off, the whole frame is out of alignment. The glass doesn’t fit. The mat is off center. Mistakes compound, and it isn’t until you assemble all the pieces that you even notice the flaws.
You could try measuring things more carefully, but the best solution is marking instead of measuring.
Making your cuts based on the size of the other pieces of the work. Your plan may call for a 20″ board, but really it needs to be the length that fits with the first board. Starting with a plan, but grounding it in reality.
A plan helps you figure out where to start, just like a cut list helps a woodworker orient their project, but once you get started you need to be willing to adapt the plan to fit the current situation. Otherwise you may very well find you’ve got some gaps in the final product.