Depends on the type of stain and wood. I would recommend you sand the original stain off (this should take 5 - 10 mins with a power sander and some 120 or 240 grit paper) then wipe clean with a cloth...
I can't verify this info, but a Hm Depot paint dept worker said:
use up to 1/4 pint (4fl oz) of water for water based stain per quart
- or paint thinner/mineral spirits for oil-based stain
Wood stain is abosorbed into the wood, so the wood takes on the color of the stain. This is why the features of the wood are still visible, unlike with paint.