To start with, you need a small carton of buttermilk – 1-2 cups and milk (I use full fat milk) 
Pour milk into a bowl 
Add buttermilk:  use 1 cup of buttermilk to 1 liter of milk, mix well 
Cover the bowl and leave at room temperature for about 24 hours (or overnight if very hot)
Check for consistency – once fermented it should be slightly thick and smell pleasantly sour 
Use beaters or whisk and mix for a few minutes until smooth and free of lumps 
Pour back in a clean milk plastic bottle or any container you can easily pour form. 

NOTE: if you pour the buttermilk back into original milk bottle, label it as buttermilk to avoid confusion or someone can get sour over a sour coffee! 

Store the buttermilk in the fridge – when cold it will thicken a bit more. 

When you almost have used it all, save the last cup or so and repeat the process – it’s like a bottomless jug of buttermilk! 

I have repeated the same process for months using only one carton at the start. I have tried the same method with kefir and it worked. 

Buttermilk keeps 2-3 weeks in the fridge, but check regularly. Make the quantity according to your consumption and it’s better to make a fresh batch more often. If you made 3 litres for instance and used more than half, pour what’s left into a smaller container. 

If the buttermilk starts to smell unpleasantly sour, discard and start with a new batch form beginning. 
Also, by buying organic buttermilk for a start and organic milk first every time after, you are making your own organic buttermilk – and you can save considerable amount.