Put both ingredients in small sauce pan. Bring to the lowest simmer you can achieve.

Using a muddler, smush berries heavily into water. Liquid will turn purple. 

Continue simmerings until liquid decreases and concentrates. (I let mine simmer for about 10-15 minutes.) 

Stir every minute or so, smushing berries further using the back of your spoon.

Remove from heat and let cool.

Place fine mesh strainer over a bowl, and strain contents of pan into bowl. Using the back of the spoon, get as much liquid as you can out of the berries. (You may benefit from letting the strainer sit over the bowl, unhindered, for another 10-15 minutes.) The remnants inside the strainer should be very thick and pasty. 

Put concentrate in small storage container and refrigerate.