I am trying to figure out how to locate the balusters on the bottom rail. The top side has mitered holes that they fit in. Instructions say to screw the baluster to the top side on the bottom rail. What is the trick to accurately mount (spacing, center pilot hole)?

Cut rail to length to fit between post. Note that in drawings the top of rail is red, bottom of rail is black. Top end of rail is marked. The dotted line is just a theoretical plumb line showing proper angle of pilot hole and baluster.

Turn rail over so that bottom edged in on top. Turn rail around so that top end of rail is at bottom. Lay rail on steps. Mark proper spacing. Do you know how to make spacing come out even?
Drill pilot holes with hand drill that has a level bubble.

Do you know how to find proper angle to cut ends of balusters?