Measure the existing banister's length that extends from the top of the staircase to the bottom of the staircase. Measure the width and depth of the existing banister.
Select a prebuilt banister. Choose the style that complements the surroundings and can fit the measurements for your stairwell. If a banister is not available in the same length as your existing banister, buy one that is longer and trim it to size with a table saw.
Make your own banister as an alternative to buying a prebuilt one if you want a distinctive look. Trim a piece of wood with the requisite width and depth to the length of your previous banister. Use a router to make a groove that corresponds with your spindles.
Remove the existing banister. Use a chisel to dislodge the banister from the spindles. Sand the new banister to smooth out the edges.
Drill holes on the bottom of the banister that match the diameter of the top of the spindles. Add wood glue to the top of each spindle.
Place the banister on top of the spindles so the spindles enter the holes you drilled. Apply pressure to the banister to make it fit into place.