Put a cup or so of saskatoons into a small saucepan and add a splash of water (they aren't very juicy at first) and a shake of sugar - about a third of a cup, or to taste. Simmer until the berries pop, then cool and refrigerate until cold.
To make your milkshake, you kind of have to play it by ear - how much milk you'll need will depend on the density of your ice cream and the juiciness of your saskatoons. Put a few scoops of ice cream into the blender, then add a good pour of saskatoons and their juice, and a glug of milk, and pulse until it's as smooth as you can get it, adding extra milk or berry juice if it's too thick, or more berries if it's not purple enough.
Serving Size Serves as many as you want to.