Put 2 or 3 small plates/saucers in a freezer for "wrinkle test" (checking whether your jam has reached setting point).
Wash and sort your blackberries, getting rid of any mushy ones or those that have seen better days.
Put blackberries in a large pan or preserving pan if you own one and mash them up a bit using simple potato masher. This will release the juices and make them cook quicker too.
Add sugar and lemon juice and stir well.
Simmer blackberry jam over low-medium heat stirring from time to time until all the sugar has dissolved. It took me approximately 10 minutes.
Slowly bring your jam mixture to rolling boil for approx. 15 minutes.
When 15 minutes is up, take the pan off the heat and test if your jam has reached its setting point by placing a small amount of jam on the saucer straight out of the freezer. Put that plate to the fridge for 1-2 minutes, take it out and push the jam gently with your finger. If it wrinkles and stays in place it’s ready, if it doesn’t then continue boiling your mixture and checking it every 5 minutes until you get the set. It took me approximately 25 minutes.
If you are using jam thermometer continue boiling your jam until temperature reaches 105C/221F at which point your jam is ready.
Skim off any scum that might have formed on top of the jam and transfer it into hot, sterilised jars, cover with the lids and leave to cool. You will hear the clicking noise as your jars seal and there will be no bubble in the lid as you press it which is as it should be. It may take some time for it to happen from the moment you put the lids on so don’t worry if it doesn’t happen straight away.
Let the jars cool completely at which point you can apply your labels.