How can I tell him I like him?

It's almost summer and I've had a crush on a guy for about half the year and I want to tell him before we break up for summer. Our school's sports day is next week and I was thinking of telling him then bc I could walk away if it got awkward and I'll be spending the whole day with him so I'll have time to prepare. The thing is, I don't know what to say or do! How do begin? Do I flirt first and build it up or just randomly blurt it out when we're alone and see where it goes? I'm really shy and I've never done anything like this before so I'm really nervous. The fact that I'm only about 60% sure he likes me back doesn't help. Advice would be really appreciated :) thanks