Its an old stair rail that has been staind many years ago and is now faded in some places. Once I have sanded the old stain off, how do I restain it? Do I stain first then apply sand and seal and then polly?

'stain first then apply sand and seal and then polly'

I'm not sure what you intended to write there. Stain doesn't usually wear off because it soaks into the wood grain, so I suspect that your railing may have a tinted varnish that wore off, rather than a stain first. Matching just the worn parts probably won't work, so you need to sand it all down uniformly. Then wipe all sawdust off with paint thinner and apply tinted poly. Save all the steps of staining and then putting on poly. There's no need for a 'sealer' with poly. Minwax makes many tinted finishes.