Drill pilot holes and drive screws down through stair treads and into risers. Sink the screws below the surface and fill the holes with wood putty or wood plugs.

Tap glued shims under loose treads. Use a block to prevent splitting, and drive the shim just until it's snug. When the glue has dried, cut the shim flush with the riser, using a utility knife.

Use quarter-round molding to reinforce the joints between treads and risers. Drill pilot holes, and drive finish nails into the riser and tread. Set the nails with a nail set.