Glue some 100 grit sandpaper to a flat surfase (plate glass is good, or a piece of wood, if really is flat, but make sure because most are not, and it must be PERFECT). Make sure you keep the saddle exactly at a 90 degree angle to the surface.
Do NOT, under any surcumstances, cut or plane the thing. You WILL take too much, and it will not be even enough.
FWIW, we use a big belt/disc sander for most of the material removeal.
As far as how much you want to take off, measure how much you want to lower the action at the twelfth fret, and double it; that's how much you need to remove. Draw a line, and sand to it.
The thing is, it is probably not just the saddle which is off. You need to get the neck bow just right first, and then the nut needs to be adjusted before you can even start thinking about the saddle, as both of those things will effect the action, and if you cut the saddle too low, you need to drop in a shim, which is fine, unless you happen to have a pickup in your guitar, in which case they can cause some real problems. Your best bet is really to take it to a good repair shop and have it taken care of there.
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