Newel posts come in two main varieties; solid turned newels which are typically mounted with metal L brackets and box newels which require a wooden block mounting bracket to be fitted inside the base to anchor them to the floor. Careful measurement before cutting your bracket and securing the bracket firmly to the floor are the keys to success for this project. Fit your stair rail in place before mounting your newel to ensure a proper connection between stair rail and newel post.
Turn the newel post over so that the open bottom is exposed. Use a tape measure to measure inside the bottom of the base from inside to inside. Measure each direction at each corner and in the centre of the newel. Write down the smallest measurement in either direction.
Use a carpenter's square to mark out a square on a piece of 2-inch thick lumber the same size as the smallest dimensions of the inside of the newel post. Keep all corners square. Cut the mounting bracket block out with a mitre saw to ensure square corners.
Fit a 1/8 inch bit into your cordless drill and tighten the chuck. Measure and mark the mounting bracket one inch in from each corner in both directions. Drill pilot holes in each of the four locations all the way through the block with the 1/8 inch bit.
Locate the correct position and alignment for the newel post on the floor or stair and position the wooden mounting bracket centred on it. Drive one 3-inch drywall screw down through the bracket into the floor through each of the four holes until the block is firmly pressed to the floor.
Fit the hollow base of the newel post over the wooden mounting bracket and tap it down until its bottom is even with the floor. Drill two pilot holes in each face of the post, 3/4 inches up from the floor with the 1/8-inch bit.
Drive one 1 5/8-inch wood screw through each hole in the newel into the mounting bracket. Run the screws in until the head of the screw is flush with the face of the newel.