Draw straight lines on the bottom of the newel post, connecting the opposite corners with each other. Drill a pilot hole where the two lines intersect, which is the dead center of the post.
Hold the mounting bracket over the bottom of the newel post, lining up the center hole in the bracket with the hole you drilled. Drive a screw through the bracket’s center hole and into the newel post, stopping once the screw head is flush with the mounting bracket.
Measure the thickness of the rosette and the length of the rail that will sit between it and the newel post. Measure the same distance from the wall where the rosette will sit and along the stairway. Position the newel post so that its back edge is on the measurement line.
Hold a torpedo level against the sides of the newel post, ensuring that it sits level. Drill pilot holes into the floor where the bracket’s mounting holes sit on the floor. Drive screws into the pilot holes, stopping once they anchor the newel post securely.
Position one end of a long level at the center of where the handrail will sit against the newel post, using the top edge of the level. Place the other end of the level against the wall, with the level sitting plumb, and mark where the top edge of the level meets the wall.
Position the center of the rosette over the spot you marked on the wall. Insert a toggle bolt through the mounting hole on the rosette, pushing on the bolt so it makes an impression on the wall. Remove the rosette. Drill a pilot hole where the bolt marked the wall. Place the rosette on the wall, lining up the hole in the rosette with the hole you drilled.
Insert the end of the toggle bolt through the rosette and into the hole in the wall. Tighten the toggle bolt with a screwdriver, stopping once the bolt head sits snugly against the rosette.
Remove the screws holding the newel post’s bracket in place, then lay the newel post on its side. Tighten hanger bolts into the mounting holes on the newel post and the rosette, using a pair of vice grips.
Slide one end of the handrail over the hanger bolt on the newel post. Insert the washer and nut on the end of the bolt from the hole on the underside of the rail, tightening the nut on the bolt using a box wrench.
Position the other end of the handrail on the hanger bolt anchored in the rosette. Drive the screws through the mounting holes in the newel post’s bracket and into the holes in the floor. Install the washer and nut on the other end of the handrail the same way you installed the first washer and nut.