There are a couple ways to clean mushrooms, to remove the dirt and grit that tends to be stuck on the when you purchase them from the grocery store. If you are only doing a handful, wiping them off with a damp cloth works well.
If you need to do a lot, or you were very short on time, there is a quick and easy way to do it. Simply pop your mushrooms into a colander and using the spray on your tap, instead of a steady stream, give the mushrooms a good rinse while swirling around in the colander to ensure all sides of the mushrooms get washed and to help remove any loose dirt. Pat dry if you prefer, but they can be used immediately as it is.
Do note that you will have about a 15 minute window before the mushrooms start to turn soggy from the water. So if you need to do it ahead of time, using the damp cloth method is your best bet.