The amount of work this will take depends on a lot things. Fortunately the steps are quite basic. The extent you take each step depends on the actual condition of the door and its present finish. You will have to use your own judgment as you progress from one step to another.
Matching wood stain (wiping stain).
Marine Spar Varnish (recommend satin sheen).
Good quality white china bristle brush (2 1/2 inch).
Sanding materials; sandpaper, sanding sponges (2-3), palm sander or random orbital sander.
Steps to Refurbish Your Wood Front Door
1) Remove the door handle and locks. Plus anything else that can be easily removed.
2) Move the door to a shaded comfortable area and place on padded saw horses.
3) Sanding is the next step and will dictate the following steps. If the original stain is still in good shape then lightly sand the door with 150 grit sandpaper or a fine sanding sponge. At this point you don't have to strip the door.
If the door needs a complete restaining then a lot more sanding is needed. For a true restaining I recommend sanding to new wood. Use 120 grit paper or sanding discs and either a palm sander or random orbital sander. The 120 grit will go through the existing finish and stain quickly, be careful to sand evenly. Progress through 150 then 180 grit. This will make the sanding scratches so small that they won't show when staining. I like to do the final sanding by hand and in the same direction as the wood grain. Make sure to remove all sanding dust before proceeding.
4) Stainable wood filler is needed for any cracks. Apply 2 coats to large cracks. Sand the filled areas to remove any excess
5) Now it's time for the wood stain. If very little sanding was necessary then this will help to blend the color of the door. Wipe on the stain until an even color and look is achieved. If the door was stripped clean then apply a prestain wood conditioner before applying the wood stain. This will help prevent color blotching.
6) Rehang the door and allow to dry overnight. If the door isn't dark enough from the stain apply a second coat while the door is hanging on its hinges. Allow to dry before proceeding any further.
7) Apply 2-3 coats of spar varnish for a good protective finish. The first coat is the seal coat and will soak into the wood a little. Lightly sand each coat before recoating with a fine sanding sponge after 24 hours. Remove the fine dust with a vacuum and tack rag.
Spar varnish dries slowly. You can add a splash of Japan Drier to help it dry faster, set up quicker. It will still need 24 hours between coats. It is best to apply the varnish in the morning so a dead bolt can be carefully installed in the evening. The finish will still be soft but this is manageable.