There are several ways of drying mushrooms:
Important is that mushrooms should always be out of reach from children and pets.

1. You just need a towel. Put one towel on a table, or somewhere where the sun doesn't shine directly on it. Cut the mushrooms in half (from top to bottom). Put them on the towel and make sure the mushrooms do not touch each other. You can put it near a heat source but not too close. The mushrooms will dry in two weeks, depending on the room humidity. Make sure to change the towel once every few days, to prevent it from getting too wet. This might cause the mushrooms to rot. 

2. Put the mushrooms on a table or towel. Use a black t-shirt or a black piece of cloth and cover the mushrooms. Now hang a lamp from 60 watts about 20 cm above the t-shirt. The mushrooms dry within a a few days.

3. You can put the mushrooms in the oven with a maximum temperature of maximum 60 degrees Celsius for several hours. Do not use a microwave!

These are a few simple methods. There are countless more, but they often involve chemicals, dry ice and other things most people do not have in or around the house. When you try to bend the mushrooms and they break instead, you know they are dry.