Unscrew the nuts under the back of the existing toilet seat that hold it in place. Seats often have a lidded compartment that houses the bolts. It should open easily by hand, or use a screwdriver to pry it open. Most seats are held in place by plastic nuts and bolts that are easy to loosen by hand. If you can’t turn the nuts by hand, use pliers to loosen them – you also may need to use a large screwdriver in slotted bolts to keep them stationary while you unscrew the nuts.
Remove the old toilet seat. Place the riser seat on the rim of the bowl, moving the seat as close to the front edge of the bowl as possible. Align the bolt-placement holes in the new seat with the bolt holes in the toilet bowl. A riser seat typically has several sets of bolt holes so the seat can be adapted to fit many brands of commodes.
Insert the bolts through the riser seat and the toilet bolt holes, and tighten the nuts underneath to secure the seat. Some types of riser-seats have a bracket that you fasten onto the commode with the bolts, and then the seat slides onto the bracket.
Attach rim brackets provided with your riser seat -- if you have a rim bracket system instead of the bolt holes -- by positioning them over the bowl rim and tightening the wing nuts until they grip the rim. Fit the riser seat over the brackets and finish tightening the nuts until the brackets are immovable.