There are many ways you can cleanse your crystals and Malas, so just use the method that calls to you.

Option 1: Moonlight
This is my personal favourite. When there is a full moon, place your mala or crystals outside under the moonlight or in a window where the  moonlight shines.

Option 2: Water.
Bathe your crystals in water, even better if you use moon water (water charged by a full moon). *Please be aware not all crystals can be put in water, so please google it first*

Option 3: Earth
Place your crystals on the earth or in the soil overnight.

Option 4: Sound Vibration
Place your mala or crystals next to a crystal or Tibetan singing bowl. Create the angelic sounds for at least 10 minutes.

Option 5: Selenite 
Hold your selenite and put your intentions into it about cleansing your other crystals. Then rest your crystals on the selenite for 24 hours to allow time for it to draw the negative energy out of your crystals.