It is not difficult to effectively reinforce a creaky stair banister. You can do so by following these steps.
Step 1 - Use Anchors
First, if you have access to the banister from beneath, it is a good idea to drive a series of anchor screws into the base of the banister rails.
Step 2 - Use Wood Glue
Next, using good wood glue, determine the spots where the creakiness is the worst and insert wood glue underneath the rail to set the banister.
Step 3 - Use a Dowel
If needed, provided you have access from underneath, you can often fix the creakiness with a wooden dowel, a drill, and glue. Be sure to use good wood clamps to ensure that the creak doesn’t come back.