how to tell my guy friend that I have a crush on someone else?

Just say "no thank you" and let him know that you feel a friend connection with him, but not that sort of dating attraction. (word whatever way you think is best).

Don't tell him about your crush. That would be like telling him this other guy is better.

Most people do not understand the magic of attraction. We are either attracted in a way we want to date someone, or we are not. Can't force attraction at all.

There's really no need to tell him about a crush until such point when that crush becomes a relationship. Just telling you fancy someone would be torture without a reason. If you actually start dating this person that's when you'd tell him you're seeing someone.

You don't. Just tell him (when he asks or when you have to) that you're not romantically interested in him. That is all that matters to him. You don't want to make it seem like he is less than someone else, he is just not for you, that's all.

If he doesn't ask, and it would be appropriate to talk about future or relationships you both have, or if this is the kind of information you share with each other, you can just say you went on a date with someone.

if he asks you out tell him your sorry but you like someone else and also that you still value y'alls friendship and want to stay friends.