After starting the process by mixing my dry sourdough with flour & water, the mixture had been inside the proofing box for 24 hours.
At 11PM on Tuesday, I did the first "feeding"...adding 1 cup of flour and 3/4 cup water. Here is a picture before the feeding (nice and bubbly!): On Wednesday morning around 9AM, I did the second feeding. Our jar was getting a little full, so I poured out half of the mixture into a bowl and stuck it in the fridge as a backup. I then stirred in 1 cup flour and 3/4 water. Hopefully we will start seeing some frothing soon!