You will need:
Sponges in various colors - 3 sponges per water bomb
Elastic hair bands
Cleaning cloths (optional)
For each sponge bomb you will need 3 sponges in various colors. A hair band and a few strips of dish cloth if using.
Cut each sponge in strips lengthways.
Cut cloth into similar size strips (we did not completely unfold cloth to cut so once it was assembled we could fan out multiple layers of the cloth).
Stack the sponges and strips of cloth three high and three wide. We had the cloth in the 'middle of the sandwich'.
Tie elastic band around center of sponges.
Fan out the sponges and cloth if using. The first is without the second is with.
Now take them outside and be prepared to get wet.
Here is a picture I found showing the water bombs last year, I filled up a small paddling pool with water so the little ones found it easy to 're-load' for water fights. The photo is not great but you get the idea.