The basic process is the same between recipes (eg https://www.seriouseats.com/homemade-yogurt) but it might take a couple of failures and tweaking (take notes) until you have something that works for you:

* Buy a food thermometer (also super useful for baking and cooking meat!)

* Heat milk to 85C

* Hold between 85C and 90C for 5 mins

* Cool to 50C

* Add a spoon of yogurt (make a new batch before the old one runs out!)

* Keep above 35C for 3-12 hours. You can use the oven with the light on, wrap the yogurt in a blanket and put it in the hot water cupboard, or I like to put it in a cooler alongside 2x as much boiling water in another jar.