Here is a short instructable that will show you how to turn an old or unwanted wooden ski into a longboard.

Supplies

Wooden Waterski (can be found on Craigslist or eBay)
Longboard Trucks, Bushings, Hardware, Wheels, Bearings, Etc. (or Complete Kit)
Bench/Angle Grinder (optional but highly recommended)
Wrench/Sockets to fit Hardware (or Skate Tool)
Measuring Tool (a ruler will suffice)
Drill and Drillbit
Phillips Screwdriver

Step 1: Prepare the Ski

Take all accessories off of the ski, such as the fin and binders. Binders and the fin can usually be unscrewed, but the fin may have to be cut off if it cannot be removed by other means.

Step 2: Decide Where You Want the Trucks

Lay the ski upside down and place the trucks on top. Move them around to see where you want them to be on the board. Make sure not to position them too close together or too far apart. 38-45 inches apart should suffice. Once you are happy with their placement, mark their positions on the ski with a pencil at the front and back of each. Remove the trucks and measure their width. Measure the width of the ski at that point. Subtract the width of the trucks from the width of the ski, and divide the resulting value by 2. Measure this far in on either side and make a mark on the lines you've already drawn. These will be the corners of where the trucks go. Do this for both the front and back, because the ski will likely be different widths in the different spots.

Step 3: Mark and Drill Holes

Place the trucks in their marked positions and use a pencil to mark the holes where you will drill into the ski. Trucks often have more than four holes, so use whichever four you want. Do this for both sets of trucks. Then drill the holes, making sure they are large enough for the screws to go in, but not too loosely.

Step 4: Install the Trucks and Wheels

Put the trucks, wheels and other parts together if you haven't already. Here is a video about how to do this. Install them on the ski with the screws and nuts.

Step 5: Preventing Wheel Bite (Optional But Recommended)

Wheel Bite is when the wheel contacts the board while turning and causes the longboard to slow down or stop abruptly. This can throw the rider off the board and cause injury, so be careful. With the wheels on the board, stand on it in place and turn the board from side to side. Note and/or mark where the wheels touch the side of the ski. These areas can be ground down with the angle/bench grinder to help prevent wheel bite. Another method for preventing wheel bite which can be used in conjunction is tightening the kingpin nut (the big nut that faces down on the trucks), but this will decrease your turning ability. You can experiment to find the right balance of maneuverability and safety.