Steve. literally the more you do the better you will get. I like to start with a rounded overlay and thin it down as much as I can before I glue it, but you still have to sand.
I have gotten real good at shaping with a file. Have you ever used one as a draw file ? Instead of pushing the length thru the target, you hold the file with both hands, one on tip one on tang, hold it sideways to the target piece and lay the file on, in the center of the file, then push and pull so the target piece crosses the narrow (width, not thickness) part of the file, not the length. Take it easy and you can learn to really control it to the Nth degree.
Then of course sand it. Sand right on into the glass when you feather it in, you won't hurt it if you take it easy.
ChuckC