Remove the pins from the door, using a flat head screwdriver. Remove the hinges and other hardware. Set the hardware aside.
Go over the door lightly with 180-grit sandpaper, working with the grain of the wood. Do not sand through any stain or other treatment on the door.
Wipe the door down with a tack cloth to remove all dust.
Pour the varnish through a paint strainer and into a paint cup. Varnishes typically have sediments that must be removed prior to application.
Brush an even coat on the door with a varnish brush. Go with the grain of the wood. Allow the varnish to dry for 24 hours.
Sand the first coat on the door lightly with 180-grit sandpaper, going with the grain of the wood. Wipe the door down with a tack cloth.
Apply a second coat of varnish. Allow the varnish to dry for 72 hours.
Reattach the hinges and other hardware, using a screwdriver. Hold the door in place in the door opening. Line up the hinges and insert the pins in the hinges.