Put the cherries in a strainer/colander and run them under tap water to clean them.
Spread out a towel on a flat space. Shake the strainer to remove excess water, pour the cherries onto the towel, and pat them dry.
Use a cherry pitter like this one from OXO to remove the pits from the cherries. You can also use a skewer, toothpick, chopstick, or knife to pit the cherries, but this is messy. Do yourself a big favor and buy a cherry pitter. I can't recommend it enough.
Place a sheet of waxed or parchment paper on a baking sheet, and cover the tray with a single layer of cherries. Don't overcrowd the tray. If you have a lot of cherries to freeze, lay a sheet of waxed/parchment paper onto the first layer of cherries, then prepare another tray of cherries and stack the second tray on top of the first.
Slide the tray(s) of cherries into the freezer and allow them to freeze for at least 6 hours. Or, just leave them to freeze overnight.
Take the baking tray(s) with the cherries out of the freezer and put the cherries into a resealable freezer bag.
Gently press the air out of the freezer bag(s) and seal them.
Straw "vacuum seal" trick (optional): Slide a straw into the bag and suck out the air so the bag "shrinks" around the blackberries. Slide the bag down the straw as you inhale, and quickly seal the bag while you remove the straw.
Write today's date on a freezer label, stick it to the bag, and place the bag in the back of your freezer.