when you rip out an existing window but need to do a new construction window with a fin,what sort of Waterproof membrane or flashing do you use? is it a self adhesive that you only apply to the upper part of the opening or a bit on the sides as well?

We just installed several beautiful interior, solid, 6 Panel Pine doors in our home, and now want to stain them. The issues are: 1) whether I am capable of doing this myself? don't want to hire someone, but, I've never stained a door before, nor am I a painter/stainer by anymeans. 2) Do I need to precondition them first? I've read other threads online about this -- some say "it's a must if you want an even color", other say the "uneveness is gorgeous and part of the natural character of wood". 3) What type of stain and varnish is best, water based or oil based? How many coats? (Thinking of Golden Oak for this soft Pine) 4) Which is easiest, Doors on the hinge or off, hardware on or off? 5) For sanding, what grit of sandpaper? First sanding? after staining? After each coat of Varnish?