Basically, you start out by giving away one detail about your person, and have people guess. Later, you give away another detail. You keep doing this until someone guesses your person.
To give an example, let's just say I was thinking Grandad; first, I would probably say, "This person has glasses." After a few people guess, I would say, "This person has white hair." and so on until someone gets it.
Get it? Got it? Good. Knock yourself out! ;)