DON'T ASK HER OUT IMMEDIATELY as it may inflict some serious damage upon you emotionally if she rejects you... Test the waters first and see if she has a thing for you.. touching her arms or hands is a great start, or talk to her in close proximity if she resists, that means it's not a good sign.
The best way to tell her is to be straightforward about it. If you cut corners she might think you're just kidding, so you have to be serious about the topic in order to convince her that you see her as more than just a friend. Once you do, asking her out on a date will probably not be too difficult. Since you both are friends, the only part of the date that will be awkward isn't the asking but in actually seeing each other across the table for the first time as more than just friends.