Your friend probably senses that you treat them differently than before. Your crush on them already shows even if you don't think it does. Goo-goo eyes and other flirting. Others might have even said something to them about the vibes you're giving off. Perhaps they share your feelings. Why not find out?
Get your feelings out in the open early before the situation becomes really awkward. If they don't feel the same way about you, you will know early on and won't invest a lot of time and emotional energy worrying about how to tell them. It's okay to be rejected. It's also easier to just go back to being friends when it's discussed early on.
Just take a chance and be open about your feelings. A conversation starter might be: "Can I make a little confession?" OR "Can I get something out in the open? Maybe you've noticed I've been acting a little differently around you ... ." Wait for their acknowledgment, describe how you value their friendship but how you have also started to see them in a different light. Ask if they are open to dating so you can both see if there might be something more than friendship between you.
You've got this!