We have a broken banister spindle (see pics). What's the best way to fix this? Do I have to take it apart? Or is there a trick to this that I'm not aware of?
The simplest way to insert the new spindle is to make sure the hole in the bottom of the handrail is deep enough that the new spindle can be inserted higher than its final position, high enough to allow the bottom stub of the spindle to clear the hole in the step and then be lowered into the hole.
Once you have sized the spindle and tested it for fit, remove it, place some glue in the upper and lower holes, and reinsert the spindle, You should then pin it to the stairs with a finish nail in a 45 degree angle, then fill the nail hole with wood filler or caulk.
It may be that there's enough travel in the upper (handrail) bore that you can slide the baluster in higher than it will ultimately be, allowing you to swing the bottom end into place.
Another technique is to cut off the dowel on the bottom of your new baluster flush with the base of the baluster. Now drill a hole the size of the dowel, and slightly deeper than the cut off dowel is long.
Insert a spacer in the stair hole that will raise the cutoff dowel, keeping it protruding slightly if inserted. It should look like this:
(dowel) ___ ___| |__ (step surface) |___|
Now, insert the dowel into the baluster base. It must be loose, so that it can drop into the stair hole when you swing the baluster into place. If it isn't, sand it a bit to reduce its diameter slightly. It must also be free to insert fully, flush with the bottom end of the baluster.
Once you have a free-sliding dowel, swing your baluster into place. The dowel will drop into the step hole about half its length. Optionally (and preferably) coat all contact points with wood glue before assembly.