Use a 10 to 1 ratio. That would be to add 1 gram of DME for every 10 ml of final volume. (If you're making a 1 liter starter, add water to 100 grams of DME until you have 1 liters total.)
That's the easiest way to do it if you have a scale. If you're using measuring cups, use 1/2 cup DME for each 2 cups of water, and simply multiply x2 for a 1 liter starter.