Remove the stems from the mushrooms so the caps can lay flat.
Put one mushroom on a black plate with the gills facing down. You can also use aluminum foil, especially if you want to harvest the spores for growing.
Cover the mushrooms with a bowl or other container and wait for a few hours or overnight. If you're worried about the mushrooms getting soft, put them in the fridge overnight.
Peek under the mushroom cap and notice the color of the spores, then compare to images and field guides to assist in identification.
Notes
There's lots of different ways to go about doing this, using a black plate is just one of many examples. Remember that spore print colors can have very subtle differences, and what appears to be a distinct color to you may be slightly different to someone else. A spore print should be one of a number of identification tools you use for the ID process.