Add shaving cream and mix with a spoon, but you will likely also want to use your hands to mix.
You will know you’ve added enough shaving cream when you can easily press a snowball from the mixture and it will leave very little residue on your hands.
Notes
While I have offered approximate measurements for the baking soda and shaving cream, you really don’t have to measure. This DIY does not need to be exact like some projects do.