If you’ve removed the stain using a stripper first, you can sand back the wood using 100 to 150-grit sandpaper. Follow up with 180-grit sandpaper to smooth it out.
If removing the stain without a stripper first, you should start with 60 or 80-grit sandpaper. It’s more coarse, so it will remove more of the stain. Make sure to use minimal pressure and go in the direction of the wood grain to avoid damage.
Once you’ve done that, move up to 150-grit sandpaper before smoothing the wood with 220-grit sandpaper.