Drill holes into the floor or stairs according to the railing manufacturer's recommendations, using the drill. The manufacturer indicates the precise placement, size and depth of the holes.
Position the railing in place, with the balusters -- or posts -- sitting in the drilled holes. One end may attach to a wall -- the top end in the case of a stair railing -- so place that end flat against the wall.
Hold the railing in place. This should require two people, one on each end.
Attach the railing to the wall if it is designed to do so. The railing should have screw holes at the top of the railing, in the part touching the wall. Screw the screws through the holes and into the wall using the electric screwdriver.
Attach one of the end balusters to the floor or stairs if the railing doesn't attach to the wall. The baluster should have a cap, above the portion sticking into the holes, with screw holes in it. Insert the screws into the cap holes and screw them in place with the electric screwdriver.
Secure the baluster at the opposite end of the railing, in the same manner. Then continue, securing the rest of the balusters in any order that you desire.