Money Wash
If you've done a few heists by now then you should have a collection of marked bills.
Before being able to spend these, you'll need to either visit a ped that will wash the money for you for a cut or wash this yourself in a washing machine.
To find the ped you'll need to visit one of the Money Wash locations marked on the map. Note that they will charge a percentage cut for their services.
Otherwise you'll want to buy a washing machine from YouTool and place this in your property to stop others being able to take advantage of this.