Like most things you have become good at, you will need to practice your masking skills. I suggest getting a scrap of plexi or masonite or even metal if you can find it and masking off something on it and paint it for practice. Learn from your mistakes on scrap not on the thing you want to make perfect.
Most of what I have learned in my life has been through trial and error, masking included. I have screwed up my share of stuff. Starting over is not fun, but you have to look at it as an opportunity to learn and grow. The great thing is you can always sand it and start over. This piece below is a good example. I have sanded the paint off of it 3 times. The great thing is that even though it looked ok painted It just was not right for the case and by re-doing it I found changes that will make the case 100 times better than the original plan. It is an opportunity to learn and educate yourself.
