Cutting and Prepping
Adjust the fence of your table saw so that it is spaced from the blade, a distance equal to the height of the baseboard you want to create. Baseboards are typically 4-to-8 inches tall.
Cut enough strips of 3/4-inch thick MDF to cover the base of all of your walls. Use a push stick to push the strips through the blade, rather than run your fingers between the blade and the fence.
Release the collet of your router with two wrenches, turning the outside nut counterclockwise and the inside nut clockwise to loosen it. Fit a detail cutting bit of your choice with a bearing on it. Adjust the depth of the bit so that the bearing is 5/8 inch below the base plate of the router.
Routing and Finishing
Lay one of the MDF strips on a flat work surface, with one wide face down, and the narrow edge facing you. Clamp it down with C-clamps. Set the router on the MDF so that the bearing will ride along the 3/4 inch edge of the piece and the detail will be cut into the top edge of the baseboard face.
Start the router and run it along the edge of the MDF from left to right. Keep the bearing pressed against the edge of the board and the base plate of the router flat on the MDF. Run the router at a slow, steady space for the cleanest cut. Repeat the routing process with each board.
Sand the baseboards with 150-grit sandpaper. Apply a coat of prime with soft bristle brush, spreading it as evenly as possible to prevent runs and drips. Wait for the primer to dry to the touch. Apply two coats of latex semi-gloss paint in the same way, allowing the label recommended drying time between coats. Allow the paint to dry thoroughly before cutting your baseboard to length.