Tell her what you want.
Make your case, clearly, concisely and directly.
"You know, we've been friends for quite a long time. We seem to get along well, we're both have fun together. I'd like to take this to the next level."
Watch her reaction.
If it's positive, gather her up in your arms and plant a really nice one on her.
If it's mediocre, continue the conversation. It will obviously depend on what her issues are, but if you maintain an active listening role, suspending judgement and the need to fix whatever's wrong, you'll probably be ok.
If it's negative, tell her in some manner that you'll leave the door open if she wants to enter, but you won't bring it up again because you don't want to taint the friendship.
NOTE: Wimmen change their minds at the drop of a hat, so be flexible. She may say "yes" to a relationship today and then distance herself tomorrow. Or it may be the opposite. Just follow her cues. And if it gets too confusing to keep up with the mood swings... NEXT...