What happens next?

How to wash a backpack
Empty your backpack.
You don't want to wash any items in your backpack that could be damaged by water. Turn your backpack inside out and use a small vacuum to clean the hard to reach interior corners of your backpack that might have small particles of dirt and garbage.
Once you are finished emptying your backpack, leave the pockets unzipped. Set all of your belongings from inside the backpack in a plastic bag, so you can put it all back in after you clean your bag.