from This Old House magazine October 2013
Use a thin flat bar to gently pry off the end cap on the stair tread, which will expose the mortise that holds the baluster. Insert a shim to push the baluster tight against the banister on top. Then reattach the tread cap.
This is the first in a series on survival skills for homeowners. We can help with staircase maintanence, update and repair or any carpentry needs should you choose to not do it yourself.