Hi, Catherine. You aren't Catherine in St. Pete are you? Anyway, get yourself some saw horses. This makes the job much easier. And put some old carpeting or something on top of the horses so they don't scratch the door. Lay the door on the horses and sand to 150 grit on 4 sides--don't worry about the top and bottom ends. Vacuum and tack rag. Apply a spit coat of one part 3# cut shellac to 5 parts denatured alcohol but now include the ends. This will only take one hour or less to dry. Watch the dates on the can of shellac you buy and do not accept any that is close to 6 months old. Sand lightly, vacuum and tack rag again. Stain, if you wish, now. Apply oil based clear polyurethane to one side. When it has dried, sand lightly with 400 grit wet/dry paper done dry with a sanding block, vacuum and tack rag. Apply the second coat. When dry (give it an extra day), flip the door over and apply poly to the other side and two edges. Watch for drips and runs on the edges and wipe them off as soon as possible.
You can drill and mortise for the hinges and lockset either before you start the finishing or after the whole door is dry. I kind of prefer doing it after but you have to be more careful with tools. Remove the old door hardware and lay the old door exactly over the new door. Transfer the hinge and bolt locations with a tri-square and hobby knife. Transfer the lockset location with the drill for the lockset size--usually 2-3/8". Using a hinge leaf and hobby or utility knife mark the new hinge locations exactly. Use one of Norm's nice sharp chisels to first go around the outline of the hinge and then mortise out to the depth of the hinge. Then install the hinges, predrilling from centered holes. Drill for the lockset but just go through from one side enough for the center drill to pierce the other side. Flip the door over and finish drilling from that side. Use the lockset manufacturers template to locate and drill the hole on the door edge. Mortise for the plate, drill and install that. Install the door. Now make a cup of coffee and admire your work.