There's more than one way to slice a mushroom. But if you're working with mushrooms about the size of an egg (like creminis or buttons) consider using an egg slicer; it'll help you make a pile of consistently-sized slices in no time. Here's how:

After you rinse and dry or brush your mushrooms to remove any dirt, trim away the very bottom part of their stems. Open the blades of the slicer and orient the mushroom the way you'd like your slices to go.

Then just slice down as you would with an egg.

You should end up with nice even slices.

If you're in the market for an egg slicer, look for one with two blades -- one for slicing and one for quartering.