As with any project like this – first step is to remove the door and place it on a flat surface, where it would be easy to work. Your best bet are foldable sawhorses that would allow to move freely around the door.
Remove all hardware! This includes hinges, handles and any other metal, or plastic that is removable. If any of these additional pieces is glued, or permanently attached – consider replacing it with a comparable new item, after the refinishing is complete.
Sand all the flat surfaces. Use a random orbit sander with 80-grit paper for fast removal of the old varnish or paint. If possible – use a sander with a vacuum to minimize inhaling and other contamination. Then repeat with a more fine 100-120 grit paper. Carefully scrape the moldings with a chisel, making sure to preserve the wood shape. Any hard to reach areas you need to hand-sand. When all is done – vacuum all dust and wipe with a clean cloth.
Next choose a varnish and apply. You can use varnish brush, or a clean piece of cloth. Make sure to spread evenly and to spread or wipe any excess drips. After the first coat dries up, sand lightly with very fine 320-grit paper, clear the dust, and apply polyurethane. This will seal the wood from the elements, and will add to a fine, polished look.
After the door has dried overnight, gently hand-sand it with 320-grit paper. Wipe the dust down with a tack cloth. Brush on the second coat, following the same order, and let it dry overnight. Dust and gently sand one more time with 320-grit paper. When all is dry, reattach the door hardware.