Answer:
Long answer:
Get 400 grams of gum Arabic powder and dissolve it into a liter of water. Put your hydrometer into the gum solution and add more water until it reaches 14 degrees Baume. You then have to add about half an ounce of Listerine as a preservative. Make sure it's the regular old yellow Listerine, not mint or some other brand of mouthwash.
Real answer:
Why bother? Most of us buy it premade.