1. Combine 2½ cups (200 grams) medium-grind coffee with 5 cups room-temperature water in ½-gallon widemouthed Mason jar and stir until all grounds are wet.
2. Place vacuum attachment on jar and apply vacuum. Stop machine before rising froth reaches top of jar. Release suction by gently removing and replacing hose.
3. Repeat step 2 until coffee froth stops rising significantly. This may take 10 to 15 repetitions, depending on freshness and grind of coffee (fresher beans and finer grinds release more CO2).
4. Stir mixture and let stand for at least 5 minutes. Pour through fine-mesh strainer set over large measuring cup, then strain mixture again through paper coffee filter set in coffee cone into 1-quart container with tight-fitting lid.
5. To drink, dilute to taste (a 1:1 ratio of concentrate to water is a good starting point) before adding ice and milk or sugar as desired. If not using right away, affix lid and refrigerate.