Hm… I’ll try and offer some advice! ^^;

1. Make sure you tell them face to face, in a place where you won’t be interrupted and there aren’t distractions.

2. Ask them beforehand to listen to your entire spiel before interjecting. That way, you can come out, explain your identity, and so on before they can barrage you with questions!

3. Emphasize that your identity is a big deal to you, especially because we live in a society where you’re bombarded with sexual content at ever turn when you really don’t want to. 

4. Let them know this doesn’t change your friendship. You just want them to know because it’s important for you to have your best friend know more about you, and you trust them enough to tell them.

5. Before they get the change to ask silly questions, explain exactly what asexual is to you.

6. If they have a bad reaction or laugh it off, they are literal garbage and you deserve a million times better and also I hope they step on a Lego so that can knock some sense into them.

I hope this helps???