2 Tbsp. yogurt, this can be store bought or from your last batch, it is your starter (plain and full fat!)
Now put the lid on your jar and put your jar in a little lunchbox size cooler and fill the cooler with the hottest tap water you have up to the neck of the jar. Loosen the lid just a bit and close the lunchbox. Set it somewhere that it won’t be bothered or opened.
I did mine right before bed and by the morning my yogurt was ready. Tighten your lid back up and put it in the fridge!