How do I adjust the carburator on my riding lawn mower?

Our lawn mower is spewing out gas when we try to start it. It is an older model Murray with a B&S engine. Some friends had been helping us clean it, and they adjusted the carburator screw and cleaned the carburator, and it started just fine. The next time we tried to start it, we were having a bit of trouble so we turned the carburator screw - but we really have no idea which way to turn it to make less or more gas come out, and how you measure it (do you turn it half a turn then try it, then do it again another half-turn if it's still not right, and then again and again? which way do you turn it to make less gas come out?). Also, we know that they cleaned the carburator (that is the part that has "jets" in it, right?) - we heard one ask the other if they were sure they had put something back right-side up - would anyone know what piece they might have been talking about in the carburator and if so, how we check it to make sure it's not upside down? And could that be causing way too much gas to be coming out? Thanks in advance for any help you can give.