Good morning treechat.

Collect them, put them in a pot and put water in until all of the blackthorn berries are covered with water.
Then heat it till its cooking and filter the coloured water into another container with a linen sheet.
Fill the pot one more time with water and repeat the steps.
This time you take the linen sheet, put the berries in at the end and press everything out.
Add some winter spices and some honey or sugar.
Tastes amazing.
Same can be done with whitethorn (red berries) or you can combine them.
My girlfriend showed me this recipe. Before I only knew that you can make liquor out of them, but I am not much of a liquor fan.
We did this three times already this year.