I started with Dr. Davis' recipe. It's described in his book "Super Gut" and is also available on his website [1]. My current method is a bit simpler:

- I buy full fat milk in a glass bottle so that I can make the yogurt directly inside without having to sterilize another container. I have had more success with UHT milk than pasteurized milk.

- I add two tbsp. (~40g) of my previous batch to one liter of milk. Close lid and shake.

- Put bottle in a pot of water, submerging most of the milk bottle. I leave the lid on, but untightened so that gas can escape.

- I add a sous vide "wand" to the pot and leave it for 24 hours at 38°C (100°F)

I deviate from Dr. Davis' recipe by not adding inulin to the milk and fermenting for a shorter duration. This probably results in fewer bacteria present in the final product, but the above takes me less than 5 minutes to prepare.

[1] https://blog.undoctored.com/lreuteriyogurtstepbystep/