Lay a drop cloth on the flooring around the staircase and below it to catch debris and dust.
Remove the old balusters by cutting them in half with a hand saw and then unscrewing them from the staircase.
Drill over the current holes in the staircase left by the old balusters with a 5/8-padded drill bit loaded on a power drill, which you can rent from a home construction supply store.
Measure the length of spindle you need with a tape measure by measuring from the hole in the staircase to the underside of the top railing.
Cut the iron spindles to the correct size using a power hand saw with a metal cutting blade.
Apply epoxy to the top handrail hole. Place the spindle in the bottom handrail hole and press down. Then move the top of the spindle into the top handrail hole. Apply epoxy to the bottom handrail hole.
Clip the decorative hardware called the shoe to the bottom of the spindle and tighten it with the tool that came with your spindle set.