Mark where you want to install the base on the floor. Trace the base onto the floor so that you have an exact guide to work with.
Set up the ladder next to the tracing and climb to the ceiling. Hang a plumb bob so that it is in the exact center of the base tracing. Mark this center location with a large dot.
Install the base and column by drilling holes through the column’s guide holes for nonsleeve types. For sleeve types, secure the base and capital with two L-brackets positioned at opposite sides of the column.
Slide the column onto the base and shim the bottom until it fits tightly and evenly against the capital.
Saw the shims so that they are even with the column and caulk around the base to seal the opening.