Materials: one 6'9" 4x4, two 10" 2x4s (for the legs), two 6.5" 2x4s, two 10" 2x4s (for the feet), two 7' pieces of angle metal **You don't have to have metal, but it helps a lot.
Take the 4x4 and the two 10" 2x4s. Place each 2x4 on the end of the 4x4 and use screws to put them on to make the legs.Take the other two 2x4s and screw them on the inside on the 10" 2x4s so that they touch the bottom of the 4x4. This will cause the legs to be thicker and stronger so the feet won't break off.
Now take the two 2x4s for the feet. Turn the rail over so that the legs are sticking up in the air. Center the feet on the legs so the feet hang over evenly off the legs. Put some screws in the bottom of the feet to get them on the legs. Be sure not to make a screw go in between the 2x4s for the legs.
Putting the metal on the rail is pretty easy. Just drill a hole at the ends and one in the middle of each rail, be sure to drill beviled holes to prevent the screws from sticking up. Put the screws in the holes and put some spray paint on the rail and go skate it. If you choose not to use metal just put some wax on the top and the edges of the 4x4 so it will be slick.
This rail should only cost about $40 at the most unless you already have the stuff. You can buy the angle metal around $1-2 a foot.