The Ripstik is a new skateboard-like toy that uses, instead of four stationary wheels, two caster wheels that allow the rider to stay upright and propel themselves forward without taking a foot off of the board. Get a taste of the Ripstik's feel before you buy, by making your own out of your old skateboard.
Using a 4mm hex key and an 8mm combination wrench, remove the trucks and wheels from your skateboard. Save all hardware that you remove, as it will be used to attach the new wheels.
Having a skateboard deck with no wheels or trucks, hold the caster wheels' mounting points up to where the trucks used to be on the bottom of the skateboard. With a marker or pencil, mark where the board will need to be drilled for the mounting holes.
Using a handheld drill with a 4/15" drill bit, drill mounting holes through the skateboard deck in the places that are marked.
Using the hardware that you took off of the skateboard's trucks, bolt the caster wheels onto the bottom of the deck. It doesn't matter which way they are mounted as caster wheels will spin as the trail changes.
Set both feet on the board and move them in a "kicking" or swimming motion in order to propel your board. Push one foot forward and the other backward, then the other way around. It will take practice, but eventually you will be able to propel yourself forward and do all sorts of neat tricks.