Refinishing stairs

My stairs as starting to look warn. I have never refinished stairs before so do I just sand them down and find a polyurethane that matches the color already present?

Should I just do a light sanding? Heavy sanding down to bare wood? Stain first than poly?

Any help is greatly appreciated

 

Were they polyurethaned before? Oil or water based? Oil would have given an amber tone whereas water based would be clear looking? If there is any poly still on the stairs you will need to sand it off as new stain will not penetrate it. Stain first then poly. The stain color you want to use will determine how much sanding to do....you can try the new stain on a bare spot and also a spot that still has the old stain and see how they match color wise....this will be a good indication of much sanding to do. The sanded stairs do not have to be very smooth as multiple coats of poly will build up and give a smooth uniform finish. If you are living in the house and need to use the stairs it might be a good idea to do every other step so that you will be able to use them....assuming everybody will be able to step on every other step.
From your picture the stairs appear to have a natural finish?
Keeping the same stain color will make things go easier, especially working against the balusters, skirts, and risers.

 

With a little luck you may be able to just sand them and apply a coat or two of fresh poly. Once sanded, wipe them down with a rag wet with thinner. While the wood is still wet [before thinner evaporates] it will give you a good idea of what the wood will look like with just poly applied.

A full blown refinishing requires removing all the existing finish and getting down to bare wood but often you can get by with just freshening up the poly.