Turn the rocking chair upside down and unscrew any screws attaching the legs to the runners. Some chair legs are simply glued and wedged into holes in the runners. Remove the runners. If necessary, cut them off with a saw by cutting vertically and against the side of the mounting hole. Do not cut the leg shorter. Remove both runners.
Stand the chair up on the pegs that remain at the bottom of the legs. Sit in the chair. Place thin wood blocks under the rear legs to adjust the sitting position until the chair feels like it is at the correct angle for your comfort.
Measure from the floor to the highest side of the existing cut on the back leg. Rocking chair legs are cut at an angle that you need to correct for when you convert the rocker into a chair. Take your measurement to a piece of stiff cardboard. Draw a horizontal line at the measurement and cut the cardboard horizontally level and wider than your widest chair leg.
Place your cardboard upright and level against each side of each leg and draw the top horizontal line. Repeat for all chair legs using the same piece of cardboard while the chair is in the correct seating position. Cut each chair leg using your lines as the cut guide. Add screw-in adjustable feet to the bottom of each leg. Stand the chair up and adjust the feet so that the chair is solid.