Tear the bottom portion of the egg carton into small pieces and add to a large bowl. Cover with water and let soak until pieces have softened, about 1-2 hours.
Pour entire contents of bowl into a blender and blend until smooth.
Strain mixture in cheese cloth to remove excess water and add back to bowl. Add in flour and salt.
Knead mixture together until it forms a clay. If it feels too thick, you can add a splash of water. You want it to hold together but still be malleable.
Paint a thin coating of glue all over the interior and exterior of the bowl.
Press the clay onto the bowl and use your hands to press into an even layer. Continue until you've covered the inside and outside of the bowl completely.
Let the bowl dry completely, about 1-2 days.
Paint the inside and outside of the bowl with your color of choice.
Let dry then marvel at your handiwork!