How do you ask someone out?

You could start off by simply smiling, making eye contact and saying hello. If they notice you, you could then think about taking things further. Look for clues about their interests, as these can be a good starting point for conversations. If they seem happy to chat, you could then introduce yourself and eventually work up to meeting for lunch, a coffee, or a trip the cinema etc.

Even though it can be nerve-racking asking somebody out, don't try too hard to make an impression. The most important thing really is to be relaxed and be yourself: that way they will know your interest is genuine. Even if the person concerned doesn't seem interested, at least you will know you tried.