The title says it all.

You walk up to her, tell her that you would like to talk to her in private, then confess to her. You can give her a letter of confession as well.

You could just walk up to her and say you like her. That way you look more confident, a letter of confession can work but lacks a major boost of confidence that just talking to the girl gives you.

Privacy and lots of courage. Be ready for anything and just make sure you don,t run away before its over. Best advice i can give. Its often a little akward but its part of it don't worry.
Just don't be a dick and attempt to tell her.

Just be blunt and clear (and in a more private setting/not with her friends or other random people around). It's going to be awkward no matter what. And even if she says no, she'll still be glad you confessed.

In my opinion, the most natural way is to take ask her out on a date and create a nice experience with her. Be funny and try to not allow awkward silences to occur. After a few dates, she'll probably get the hint that you like her and then you can ask if she'd like to be your girlfriend. However, if you're still a kid and under the age of 18 and don't have a job, totally ignore what I said lol Its been a while since I've been in high school and I sort of forgot how high school confessions worked lol I never personally had to do one. I suppose all I remember from like 5 years ago is that they would always be super nice to me and if I would accept their kindness and them hugging on me, they would ask me to be their girlfriend after like a couple of weeks.