If it's someone you see often like a colleague or whatever make small talk, ask how their day is going, what plans they have for days off etc. Once you've got to know them a bit ask if they wan't to do something after work or at the weekend.
Complete stranger similar, but always going to be a bit more awkward and require more balls.
Wish I could put the above in to practice tbh it's obvious it will work if the other person likes you at all, but fear of sounding weird/awkward puts me off trying a lot of the time.
Most Helpful Girl
You can't make anyone ask you out. That's entirely up to them. Instead, you can try asking them out yourself.
1. If you are interested, then you should ask him out. 2. HOWEVER, I assume this is not superficial and that you do know something about this guy other than (presumably) he is attractive. So, that tells me that you can work on commonalities between you and him which you can explore further. Once he discovers enough commonalities between you two and there are things about you that he likes or is interested in learning more about, he will make the move.