@alliebee - It's very, very easy. This is how I do it.
Hardware required:
1 heavy bottom saucepan
Stirring implement
Ideally, a food thermometer (just less accurate if you don't have one)
Jug
Sealable container, just larger than the amount of yoghurt you intend to make
Smaller sealable container to hold your reserved culture for your next batch
Ingredients:
2pt container of milk I have used both Channel Islands and Full milk)
1 x 125/150ml carton of live yoghurt (I started with some Yeo Valley whole milk, plain yoghurt)
Optional Ingredients:
Splash cream, or couple of dessertspoons of dried milk, if you want it richer or firmly set
Method:
1. Warm your milk, in your saucepan to 185f/85c, stirring as it heats to stop the bottom catching or a skin forming on top. if you have no thermometer, heat until almost boiling. It's no catastrophe if the milk boils, but it's not strictly necessary.
2. Put the saucepan of milk to one side to cool to 110f/45c. If you have no thermometer, this will just feel warm, not hot, if you put your finger in. It needs to cool, so as not to kill the live yoghurt you will add shortly.
3. Whilst your milk is cooling, scald your containers to sterilise.
4. Once cool, stir the milk, and skim off any skin that has formed, or it may result in a lumpy finished article.
5. Empty your bought yoghurt, or saved starter culture into your jug. Ladle a few ladles of cooled, warmed milk and stir to fully combine. This loosens the culture and again prevents any lumps. If you are adding cream or powdered milk, do it now. (I haven't used this optional step, but if you're looking to up the fats, this could be an opportunity.)
6. Once your starter is loosened, combine the rest of the milk - in your jug if it's large enough, or in the saucepan, stirring to ensure no lumps. If there are any stir until you get rid of them.
7. Pour an yoghurt into your final containers.
8. Wrap both containers in an old towel and leave overnight in a warm, draught-free place.
9. Next morning refrigerate, remembering to save the little pot for next time around.
Mine has always set really well, and it gets more firm each day. The CI milk makes it very rich, but of course, you don't want much. Once going, it makes a week's supply of breakfasts for under £1 (plus additions and a 5 minutes fuel)
There are loads of YouTube films, employing various methods, including yoghurt makers, slow cookers and ovens, but to be honest, although it's an overnight process, the actual interaction is about 10-15 minutes in total.