be straightforward, the way you are when you interact generally. Simply say 'do you want to... " and suggest something date-like but not intense, like attending a movie. Then suggest dinner beforehand. If she accepts and if the vibe is good on the night, ask her if she'd be interested in seeing you romantically.
You have to be direct and up front with what you want. Especially because you have known each other or so long, you don't want to make her guess whether you mean this as a friendly thing or as a date. Be straight up because it makes you appear as confident.