A skeg is a brace that extends down into the water from the keel of the kayak, generally in the stern. It is used to trim the boat in less-than-ideal paddling conditions, and it can assist in tracking and increase stability. While a skeg does not offer the versatility of a rudder, it allows for solid foot bracing, which is mandatory for advanced paddling manoeuvres. Skegs come in different shapes and can be built from different materials. You can purchase premade skegs that come with mounting brackets and instructions. They are generally costly. To add a skeg of your own, wood is probably the easiest material to work with.
Cut wood into the shape of a semicircle 4 to 6 inches deep and about 1 foot across the flat section. A semicircle has a good hydrodynamic shape and can be easily adjusted based on your preferences. However, you could use any shape you like. Sand the rounded edges smooth. Leave the flat edge square.
Oil the skeg with Danish oil. Do not use a polyurethane. A urethane finish will crack under duress, allowing water into the wood. The finish will then trap the water, causing mould and eventually it will rot. Screw the male side of the aluminium channel track to the skeg's long, flat edge.
Screw the female channel to the keel of the kayak anywhere from 1 to 3 feet from the back of the boat using aluminium, stainless steel, or brass screws. Use silicone caulk in the screw holes to prevent leaking. Attach the aluminium channel straight and square with the hull. A crooked skeg will cause the kayak to track crooked.
Slide the skeg into the channel under the kayak. The pressure of the channel is normally sufficient to keep the skeg in place. If it seems loose, drill one hole at a diagonal through the wood and aluminium channel and into the keel. Insert a long screw. This will lock everything into place.
Paddle the kayak for a while before making changes. The kayak will feel different with a skeg. If it seems to drag too much, cut the skeg down in size. Remember that more is not necessarily better. Too much skeg increases friction against the water, reducing speed. If, after using the kayak, you want to change the size or experiment with different skeg shapes, you can easily remove the skeg to change or rework it. If you want more directional control, consider a rudder. However, if you plan on using advanced braces and rolls you need the stability of the solid foot bracing, so a skeg may be the right solution.
Always wear an approved flotation device when kayaking.