I'm no expert here, but get this
and read it and make sure you understand it. And save every bit of wood scrap/cutoff and be prepared to practice on them - a lot - to find out what will create the finish you want. Figure this out (by hands-on testing) before you go and put stuff on the instruments.
There's unbelieveable amounts of info in the old build threads. You have lots of reading to do. Lots.
For your first run at finishing, a wipe-on varnish will probably get the most recommendations here. Tru-oil, or a wipe-on polyurethane, or similar. Did I say you have lots of reading to do?