My biggest tip, every attempt is experience you will learn from, if you mess up this time, at the very least you get the experience of rejection which as horrible as it sounds, takes more and more of the sting out of future attempts, which overtime will remove some of that fear of rejection.
Be upfront that you like them in that way, but don't go too far, then maybe suggest some light form of activity the two of you could do. If you have a hobby in common that helps, if not the cinema can be good, or bowling, set a time and place a.s.a.p because "some time" will never come.
If you do go the note root, something quick and honest tends to be better than a letter, a letter gives you lots more opportunities to mess up, but can also be very overwhelming for the receiver. Example. My first crush I got the courage to give a note was outside of college, we,d sit on opposite walls, and a couple times I looked over at her, and she looked back. Third time I gave her a note telling her she made it hard for me to speak, she thought it was cute and it broke the ice.
Unless you're on a dating site, avoid using messages online as a general rule, it can make them more uncomfortable as it's not in a place that's easy to get away from, or that's viewed by others if that makes sense. Kind of like dark alley type vibes.
Make sure you have things to occupy your time other than them, this means if you are rejected you aren't left with nothing, but also if they give you a chance, that you don't come across too clingy.
While being rejected hurts, rejecting someone for any reason isn't always pleasant for the person doing it either, and at the end of the day it's you that brings the situation to them, so try and make a joke of it if it doesn't go well, and may mean you can stay on more friendly terms.
I've had the same symptoms as you before, so I know some of these tips may be harder than others. There's also plenty of stuff I'm not aware of, I'd also recommend Charisma on Command on YouTube for better advice.
