Find out what type of entertainment or activity that the guy enjoys and buy tickets to an event you think he might like. Casually tell him that you have tickets and that you are looking forward to going. If he seems interested, tell him you have an extra ticket and you would enjoy his company for the event.
The best way to ask a guy out is by going up to him and just asking him out. Making your friends do it for you is a really cheesy way to do it. The worst thing he will do is say no. All you can do is move on from there. There are more guys out there.