The first step is to be sure that the surface is clean. I wiped this earlier with a fifty-fifty white vinegar and water mix to remove surface dirt and oils.
Use some superfine steel wool to remove any flaking urethane and to expose surface. Use a tack cloth to remove dust and residue. When you're touching up you want to work on an entire section rather than just one area, because a spot repair will be more noticeable with the surrounding woodwork.
Gently stir your stain to make sure that the color is blended.
Use a soft cloth or a foam brush to apply the stain to the surface. Let it sit for a minute or so and then wipe any excess with a clean rag. You may have to apply another coat to get the stain to match with the surrounding areas.
Once the stain has dried, apply a coat of polyurethane. Make sure that the sheen that you choose matches the existing woodwork.
Allow the poly to dry, and then wipe the entire area down with some superfine steel wool and apply a second coat. Then repeat the process. I'm Mrs. fixit and it's just that simple.