Well, there are a number of things you can do.

First you can send it off to someone and have them paint it.  It will cost you a pretty penny but it will be done right.

You could try doing it yourself too.  More work but the cost is less depending on what materials you already have.  I'm going to guess that you don't have an airbrush or compressor.  That leaves you with using spraycans.  From what I've read people have had good luck with Krylon spraypaint. 

Then you need to ask yourself what kind of Camo do you want to do?  You want tiger stripe?  Digi? Standard BDU Woodland? Multicam?  Once you determine what kind you want to do you have to figure out your colors and whether or not you want to buy paintmasks (highly suggested especially if you want to go Digi like I did with my ACU...see sig picture) or make your own.

I also suggest checking out tacticalcamo.com as he has some good write ups over there on camoing your marker.  I used his templets to paint my marker and used Duracoat paint with an airbrush.  Was easy to do and came out looking good.