Yep, practice, practice, practice :-) I have made just about every mistake it's possible to make, but you learn something every time. Most mistakes are correctable, it doesn't have to be the end of the world cos you screwed up. You can't learn to do it right till you've done it wrong at least once :-)
I still mark the pieces, even though archtop sides are parallel.. I saw a flattop built front to back - had the visual effect of looking through a telephoto lens! Sounded fine.
Grahame mentioned correctable mistakes, which goes back to my "if you build them, you might repair them". And mistakes are not the end of the road, just a pop quiz.
But I sure like to make it only once, if at all, and learn from it.
Practice makes perfect-- I once wanted to learn how to carve a neck for a guitar that I was building -- soooo---
I went down to my local lumber supply and bought up some 2X4's --- took the things home and proceeded to carve them up.
It took me 2/2X4's before I got the hang of it but instead of spending lots of cash (which I didn't have anyway) I finally made my first neck---- It was a good experience and I enjoyed my self... moral of story, use the cheapest thing that you can find before you do it to the good stuff....
Peace, Donald