Measure over at least 1 inch from the side of the lowest stair tread using a tape measure, and draw a small pencil mark on the tread. Place a yardstick on the mark so that the stick runs back to front on the tread. Draw a pencil line along the entire length of the yardstick on the tread.
Read the instructions for the railing that you are installing to determine if you will have one, two or three spindles per tread. If you only have one, measure the tread from back to front and divide it in half to locate the spindle location, which will sit on the 1-inch line you just drew. If you have two spindles, mark one hole at the front and one in the middle of the tread. For three spindles, mark one hole at the front, a second one a third of the way back and the third hole two thirds of the way back.
Repeat the process on each stair tread to mark the location of each spindle.
Install a 1-inch-long drill bit into a drill and don safety glasses. Position the drill bit over the first pencil mark on the lowest stair tread. Keep the drill bit perfectly perpendicular to the tread and drill down 1 inch. Repeat the process to drill 1-inch-deep holes at the location of each remaining spindle mark on the stair treads.
Turn each spindle upside down and drill an identical 1-inch-deep hole in the bottom center, using the drill and 1-inch bit.
Cover 2-inch-long wooden dowels with wood glue. Tap one end of the dowels into the stair tread holes and insert the holes in the spindles over the top of the dowels.