Drill a 1/2" hole through one side of the smaller piece of hardwood. The hole will hold the 1/2" dowel. On the same side of the small block as the 1/2" hole, drill the block into place on the center of one long edge of the 24" piece of hardwood.
To drill the third hole, remove the dowel, and move the work piece down so that the dowel fits inside the second hole. The drill bit will automatically be aligned with the correct spot for the third hole. Continue repeating step 7 to drill as many evenly spaced holes as desired.