Pour off as much of the dark hooch as you can.
Discard most of the starter (you can either just remove it from the jar or place some of the starter into a clean jar).
Feed the remaining starter with 100g of flour and 100g of water. Let it sit out at room temp for around 12 hours.
Take 50g of starter from the jar and feed it another 100g of flour and 100g of water. Leave the starter for around 12 hours.
After this second feeding it should double. If it does then it's ready to use. If it doesn't, repeat step 4 every 12 hours until it does.
