Leave the banister in the room where you will install it for at least 24 hours. This allows the wood to expand and contract as necessary.
Mark on the walls roughly where you want your banister to begin and end.
Measure the height of the banister rail itself.
Subtract the height of the banister rail from the height that you want the banister to have. According to Safety Is Us, standard handrails should measure between 30 and 34 inches high, so determine how high you want your banister to be and subtract the height of the rail from this figure. This is measurement A.
Measure up the wall the distance of measurement A and mark it with a pencil, matching these dots to the marks you made before as an estimation. These are the dots to use from here on out.
Measure the distance between the two marks using a measuring tape. This is measurement B.
Cut a length of banister using measurement B as a guide. At this point, the banister is ready to be stained and mounted.