Click the glass to change the flavor. Click the straw to change the color. You can move the straw off if you don't want it. Drag the other toppings on if you want. The sprinkles and chocolate drizzle are traditionally meant for the whipped cream... but do what you want. Same with the cherry. The weird dots shaped like the cup are things you can put in the milkshake.
slo-mo-jo helped a little with the whipped cream and mint leaf. Thanks! All of the art was made by me, though.