Procure a baluster to match the staircase. In this project, we used 1" x 2" clear fir boards. Cut the balusters to length to match the originals.
Look at the existing newel post base as a model to make a new one. Determine the radius by carefully measuring the existing one on the floor.
The existing newel post in this project was made of four boards assembled into long box shape. That hollow box fits over a 4-inch by 4-inch post to create a solid base for the newel post. Your newel post may be solid and require different joinery methods. For a hollow type newel post base, cut thick plywood boards into two radius shapes to match the curve of the existing newel post base.
Assemble the two radius shapes, matching the height of the existing newel post.
Cut a square hole in the top radius shaped plywood to fit a short 4-inch by 3-inch post. Check the position of the existing newel post and make sure they match. Insert the post into the new base and plumb the post, making its position on the bottom radius shaped plywood.
Drive screws in from the bottom of the bottom radius shaped plywood into the bottom of the 4-inch by 4-inch post. Position the entire assembly against the existing staircase, then fasten the assembly to the floor with screws.
To fit the bottom tread on the newel post base, position the newly cut tread in place. Cut the tread so it fits halfway around the 4x4 post on the new newel post base. It doesn't have to be perfect, since the finished newel post will slide over the top of the 4x4 and cover where the 4x4 and the tread meet.
Use the plate joiner on edges where the two tread sections meet. Insert biscuits and secure them using bar clamps.
Leave the end spindles for last, so you'll have room to attach the handrail to the newel posts or wall. Flip the handrail over and attach the handrail in place; between newel posts or between a newel post and the wall.
Place the floor nosing piece at the edge of the staircase landing. Nail through the bottom of the nosing to fasten each spindle in place. If the nosing on the staircase is already in place, you can toe nail the spindles to the nosing or use a wooden dowel and glue to make this connection.