If you are at this section, you either are just here to learn for future use. Or, you are here because you have a lander within the Mun's gravity with a decent amount of fuel and are ready to begin your decent. Otherwise please scroll up to get more info on reaching this point.
Once you have made it to the Mun's influence, please make your orbit circular.
Aim yourself Retrograde and begin firing your engines.
What you are now doing is called a "suicide burn" and though it isn't the most efficient form of landing, it is by far the best if it is your first time.
Your orbit will slowly start becoming an arch.
Once your arch becomes a straight line, cut your engines. From here you will fall directly downwards, and all you need to do is keep it that way. To make sure your landing is nice and safe, periodically slow yourself down to about 150m/s as you fall. Take it slow and don't slow yourself down too much, or landing will take longer, and you will waste fuel.
IMPORTANT: The altitude meter will not say 0 at the surface. 0 indicates the lowest point on the surface of the Mun from it's core. Chances are, where you will be landing will be around 1200m
Keep yourself falling straight by firing your engines slightly to move the retrograde marker around. The retrograde marker will always move away from where you are aiming, when firing, so imagine your pushing a magnet around with another magnet (I hope I'm not the only one who has ever done that.)
At 3000m you need to make sure you arent above 400m/s, or you will probably crash, try to stay around 150m/s through this part of the decent.
At 2000m you need to slow down to between 40-70m/s. Maintain this speed until you can see your shadow, at this point fire your engines full blast until you are at 3-10m/s. Slowly let yourself fall, and when you go over 10m/s, fire your engines breifly to keep yourself around 7.
Continue downwards until you are practically on the surface, and cut your engines.