If you left some feed from your last batch of feed in the bucket (you should do that each time) you don't need to add anything. Just stir the old feed into the new feed mix really well. The way I do it is I leave a couple of inches of the old feed in the bucket, add some warm water and stir that old feed up really well. Then I start adding my new feed and stir as I go until it is all mixed up. In about 8-12 hours you will have a new batch.
You don't have to use anything to start it, but it just takes longer. Some use ACV as a starter. I use buttermilk to give the first batch a quick start.