Simple. Get references first. Second, do your homework by reading what I have to say. I started woodwork and finishing by repairing ancient spinning wheels and parlor organs as a kid. Both were in high demand in Cape Breton in the 70s and I was a super provider, I didn't charge for my work at 12-14:) Visit Isaac Smith's bad and breakfast to see a sample of my work. His historic farm is worth the trip.
I read endlessly in ancient texts on how the early American and European furniture was finished, From there I branched out into Edward Heron Allen's book Violin Making. Putting it all together, today we have the same woods, better systems for smoothing and preparing it, and a better optical understanding of what "finish" really means. Wood is at first glance a color, that's an illusion, the ingredients and structure mean it is really a color filter that returns a broad range of tone and undertone/overtones to the eye and to it's surroundings. Polishing the wood to eliminate frayed opaque fiber and sanding debri is the key. A vacuum based sanding system that keeps the wood cool is essential, heated resins (major source of color and variation) become dull if overheated.
Now, the wood is ideally as clear and optically transparent on the surface as it can be. This is really 90% of finishing. Applying a stain and rubbing in or coating with "varnish" like urethane is a way of locking out dirt, protecting from UV and oxidization both of which dull and change the photoreactive compounds in the wood.
Secondly, it provides an effect largely misunderstood but essential. A clear finish material wets the wood grain, causing it to become more transparent, very much like wetting paper. Deeper access to the inner rich colors of the wood... Then there is the surface reflection of light from the sealing coat. This has a symbiotic relationship to the underlying colors. Light strikes the surface, a portion is reflected directly, (shine) a larger portion by far passes into the wood and is reflected off off many interior resin and minerals. These bounce out of the wood at various angles providing rich character that responds differently to every light and every color in its surroundings. That's what you see in very rare old furniture and musical instruments.. Stradivarius violins come to mind.
Finally, this process has to be done during construction of the furniture or stair/railing. This is pretty obvious, one can't sand and prepare surfaces that have mating surfaces joining at right angles! A clean end to end stroke is essential when sanding and scraping the parts, Only after each part is finish ready should it be assembled.