Before you purchase a base plate, measure the base of your toilet. Use a tape measure to find the distance from one of the flange bolts to the front of the toilet and from the bolt to the back of the toilet. Not all bases on toilets are the same size or the same shape.
Removing a toilet is really a simple job. Shut off the water valve, flush the toilet and use a wet vacuum to remove any remaining water from the tank and the bowl. Disconnect the water line from the shut off valve or from the tank and remove the nuts on the closet bolts from both sides of the base on your toilet. Lift your toilet, remove the old wax ring and set your toilet aside or on towels in your bathtub.
Set your base plate over the flange and measure the distance between the flange and the underneath edge of the base plate. Purchase enough flange extenders to bring the height of the flange up to the base plate. Flange extenders can be purchased in various thicknesses, and you may need only one. Place the extender on the flange and secure it with the screws provided.
Place your base plate over the flange and, depending on the type of base plate you purchased, use a regular wax ring or a permanent reusable rubber seal that functions the same as a wax ring but never needs to be replaced. Set your toilet back over the flange bolts, replace the nuts and tighten them down. Do not overtighten or you will crack the base of your toilet. Reconnect the water line and turn your water back on.