Remove the cream two hours before you intend to add the yoghurt
Ensure your glass jar is nice and clean and has a lid to it
Place room temperature cream into the glass jar and stir in the yoghurt
Cover the jar with a lid and put it aside in a warm spot for 24 hours ... if you think you will find it hard to remember the time, then write the time to be checked on a post-it note and stick it to the jar
After 24 hours get a long handled spoon or fork and stir the contents of the jar well ... if it looks like it is thickening up as you stir then you are on track ... if it is looking quite thick then you may wish to chill it now, or if you would like it thicker continue for another 4 hours before stirring and checking again
If the mix is still quite loose you will need to leave it for another 8 to 10 hours before checking again (it can go up to 48 hours)
Once the soured cream has reached a thickness you are happy with (note that it will thicken up further in the fridge ... but after the first time you do this you will know for future how thick you will want it initially) then place the jar into the fridge and leave it overnight
Next day stir the contents and place into two smaller jars ready to be used