Dave from maple leaf tactical did a really good tutorial on this a while back.
http://thereptilehouseblog.com/
Rich from The Reptile House (you should all be following this blog, srsly) also features lots of painted guns and is worth a look at for inspiration.
As for which paints to use, I really, really rate the halfords camo paints, but krylon is also really good. If you can get it, Tamiya Ultra Flat (XF series) are the best thing since sliced bread.