Take out your kitchen scale and tare it out to zero. Place the mason jar on top of the scale and add 5 ounces of starter.
Tare out the scale again and add 5 ounces of water.
Tare out the scale again and add 5 ounces of flour.
Mix the contents of the mason jar well until it forms a thick paste. Place a loosely fitted lid on top of the mason jar and place it in a cool, draft free place.
After 6-8 hours, the starter will have doubled in size. To check if the starter is ready to bake with, place water in a bowl. Add 1 tablespoon of the active, bubbly starter to the water. If it floats, you are ready to bake. If it sinks, start from step 1 and feed the starter again.