Dont. Just kidding at least partly. Oak railings as the builder leaves them are seldom actually finished, they just have a couple coats of urethane at best over unsanded surfaces. This results in the dull lusterless finish you see, typically a dreary yellowish color. I've already mentioned that sanding is the key to this issue in another post. Sanding old railing is a different story. Hand oil and soil is present in any railing "builder finished" as the wood is largely open to the atmosphere. Then, the innocent come along and sand it down, make it pretty.... hours of hard labor. Then comes disaster: the finish applied promptly rejects the wood in a nasty fish eye way, rather like water beading up on an oily surface. This is the acid and soil still trapped deep in the wood. The labor required is wasted on two counts, one being that the quality of the railing will never be better refinished or not, and the other that it will take far more labor and thus cost to achieve a still substandard railing than replacing it would have cost!
Then, lets talk balusters. Inserting balusters is not a forbidding job. Its best done after removing old balusters and if needed drilling oversize holes under the railing. Slip the baluster down into the floor hole, then mark the side about 3/4 inch above the hole in the bottom of the railing and cut it. Make sure you cut both ends if you want a specific position for the decorative element. Apply epoxy (8x Loctite works extremely well and is remarkably strong and effective.) Don't use the 3x type, it runs and makes a horrid chemical smell that may give you a wicked headache. It's important to dry fit all the balusters then take them out one at a time and wet out the hole with adhesive. Then just slide the baluster up into the railing and back down into the floor. Align it and make sure it stays aligned for 12 hours or you will NOT be able to reset it 90% of the time. It will seem welded in place.
All that said, new balusters on builder grade railings can look fair, but they often look like a dramatic upgrade dragged down by a substandard railing.