For one thing, you have to be willing to cut out the same pattern pieces over and over and over.
Fitted shirts have a LOT of small pattern pieces.
I can think of at least four trials I made for my Inara shirt before I finally felt ready to cut into my beautiful orange jacquard. That fabric terrified me with its pristine beauty, and I wasn't going to betray it.
Test #1 - I made a drab, basic shirt pattern and learned about sewing sleeves (highly important).
Test #2 - Tested the pattern I decided to use as my actual costume base. Decided that I wanted to make some alterations to make it truer to the source costume.
Test #3 - Implemented my alterations and made sure they worked, and carefully noted what I was doing so it could be repeated.
Test #4 - Tried the whole thing out AGAIN on cheap practice satin (I should note that the first three trials were in muslin, which is a very kind fabric to a newbie).
For Rapunzel so far I have made two test skirts and three whole or partial bustiers. Below you can see some of these castoffs (in progressive order):
I used the red fabric for the latest bustier because I really wanted to try my hand at fully lining something, and it was easier with two distinct fabrics.
My trial pieces are invaluable learning experiences, and each one teaches me a lot. Alas, I am not quite finished making trial bustiers. Adding proper armholes for sleeves (in conjunction with the alterations I've already made to the bust) is a very precise and tricky business, and I don't have the fancy tools or training to do it with a compass and an equation on a drafting board. I'm sort of doing it by instinct, small tweaks, and everything I've learned along the way.
I actually spent a couple of hours last Saturday night making some of these very adjustments. As soon as I have something substantially worth posting, I will be sure to do so.
In the meantime, don't you guys just love the berber carpet in our basement? You're spending quite a lot of quality time gazing at its beautiful oatmealy speckledness!