First you have to make room in the whisky bottle for the blackberries so this is best made in the evening!
After you have had your drink add blackberries to the whisky and add sugar to taste.
The amount of blackberries and sugar is entirely personal - I usually use about two dozen black berries and a couple of teaspoons of sugar, but if you like a sweeter drink add more sugar or indeed do not use any.
Give the bottle a shake to disolve the sugar and place on a shelf in a store cuboard or pantry (in the dark). shake every day for the first few days.
Then turn the bottle every week or so and after a month or two strain and discard the blackberries.
It is now ready too drink, but the flavour improves with age.