How to use sourdough starter? A week ago I mixed 1 part AP flour 1 parts tap water, keeping it at room temperature (18-24c). I've been discarding a bit and feeding it 1:1 daily. The last few days, the next 4-6 hours after I feed it it double in size, and then it slowly goes down.

Now I want to use it to make some baguettes, the recipe calls for 20% starter. But how do I use the starter? Do I have to use it before or after feeding it? Or should I feed it and wait a few hours before using it?
The optimal time to use your starter is at peak activity.  This could range from a couple of hours after feeding to 12 hours after feeding, depending of conditions.  The top should have a slight dome, it should be bubbly...just before it begins to settle back down.