Attaching a level banister handrail between walls with one or two rosettes is a relatively simple process. First, make certain that there is backing in the wall where the rail terminates prior to installing it, typically this can be done using a stud finder. Next, before attaching the rosettes to the handrail predrill the face of each rosette with two ½” hole about 3/8” deep (or halfway through the rosette). These holes should be above and below where the handrail will attach to the rosette, leaving enough room to pre-drill and screw the rosette to the wall later.
Now determine the length the handrail needs to be at its position between the walls. Remember that not all walls are perfectly plumb so just because it fits on the ground doesn’t necessarily mean it will fit 36 or so inches above. Once you know this length, cut the rail short to include the rosettes being used, typically these are ¾” thick or 1 ½” if you are using them at both ends. Once the rail has been cut to length, pre-drill and attach the rosette to the handrail by screwing through the back of the rosette into the handrail. This is best done with two screws, toward the top and bottom of the handrail but leaving enough material so as not to split the handrail. This prevents the rosette from spinning, also a little wood glue doesn’t hurt either.
Next, assemble the balustrade and set the rail atop the balusters, this, of course, varies depending upon which type of balusters you are using. When this is done all that is left is to attach the rosettes to the wall. Predrill a hole through the rosette into the ½” hole you previously drilled. This hole should be large enough to allow the screw to “push” through rather than thread into the rosette and be angled in such a way as to ensure the screw will hit the backing behind the drywall. Your drill will be against the balusters when predrilling and screwing the rosette to the wall so using a long drill bit and a long driver (Phillips, Square Drive, etc.) will allow an angle more parallel to the balustrade. Finally, with a bit smaller than the screw you intend to use, predrill the backing large enough that you do not snap the head off of the screw but small enough that it doesn’t strip out the hole. The best way to determine the size of the bit is to use a little trial and error on a block before drilling the actual holes. After you’ve figured this out you simply screw the rosette securely to the wall, making sure you have hit backing.
Finally, to conceal the hole you can use either a ½” face grain plug or a button plug as the final touch and your balustrade is installed!