Step 1: Get your stuff together, man. You'll need hi-liters, an assortment of glass bottles, and a blacklight. You can go out and buy a multicolor pack of hi-liters for a dollar at Walmart and you can get whatever kind of bottle strikes your fancy, but I find that the best ones to use are Izze bottles.
Aside from being absolutely delicious, Izze bottles have the logo (which I think borders on chique) printed right onto the bottle rather than a label wrapped around it. This allows more light to pass through and it makes the effect look way better. Glass coke bottles work well, too.
Step 2: Pry the cap of the end of the hi-liters. Some needle nose pliers should do the trick. I would suggest doing this somewhere that you wouldn't mind getting hi-liter ink all over the place, just in case. Also, wear some gloves. I didn't in the pictures I took, and I ended up having hi-liter ink all over my hands.
Step 3: Fill up the bottle with water then squeeze the ink out of the sponge stick until the water becomes opaque with the hi-liter fluid. It's best to fold the stick in half that way you can squeeze the ink out of both ends instead of squeezing some out the top end and all over your hands. If the water doesn't become opaque after using a whole stick, add some flour to the mixture to thicken it up. Also, you might want to add some rubbing alcohol to prevent it from molding in the future.
Step 4: PROFIT.