Generally, you mash in with water that is about 11 degrees warmer than your desired mash temperature. That's because the grain is generally room temperature, and some of the heat of the water is lost raising the temperature of the grain to the desired mash temperature.
My brewery is cool, so I even preheat my MLT by adding 180 degree water to it, and letting it cool to 165 before adding my grain. Then, I stir in my room temperature grain and in my system it gets me right to 152.
Have some extra boiling water on hand, and a few ice cubes, in case you miss your temperature the first time. A word (or two) of caution, though- don't be too quick to cool or heat. Stir well first, and then stir some more. If you're just a little high, stirring more will dissipate the heat and you won't need to cool the mash. If you're just a little low, stir more to make sure the temperature is equalized throughout- check in several places. If it's different anywhere, stir some more before adding a little boiling water.
When you adjust with either three ice cubes (not kidding- sometimes that's all it takes!) or a little boiling water, go slow. Add it a little at a time, and stir it like it owes you money. Then give it five minutes or so to equalize. Many people chase mash temperatures by adding boiling water, getting it too warm, so add cold water. Then it's too cold, so they add hot water, etc.
No harm will come to the mash in 5 minutes, whether it's too warm or too cool so let it settle a bit before adjusting either way. Also, if you're like 3-4 degrees off, you may just decide "close enough", and that would probably be fine depending on where your mash temp is.
