Find a Replacement
Toilet bowls come in two sizes -- standard and elongated -- so choose a seat that fits the bowl. If your bowl is a standard rounded one -- which most residential bowls are -- you have more seat choices; it's difficult to find wood or plastic-coated MDF seats that fit elongated bowls. Unless you have a very old toilet, you don't have to worry about the bolt spread, which is the distance between the bolts that hold the seat to the toilet; it's a standard 5 1/2 inches on virtually all modern toilets. If your toilet is old enough to have a different spread, you may want to consider replacing it with a low-flush model.
Remove the Old Seat
Removing the old seat should be straightforward and easy to accomplish, as long as the bolts are in good condition. All you have to do to remove the seat is lift the flappers covering the bolt heads on the top of bowl, using a flat-head screwdriver. Hold the heads steady with a flat-head or Phillips screwdriver, and unscrew the nuts by hand. If the nuts are metal, you'll probably have to loosen them with a pair of pliers before you can unscrew them. Once you've removed both nuts, lift out the bolts and remove the seat.
Dealing with Stuck Nuts
A simple procedure can turn into a gnarly, problematic one if the bolts are corroded. An easy trick for loosening the bolts, which works in most cases, is to lock a pair of locking pliers onto each nut in turn, and turn the bolt head with a long-handled screwdriver. The locking pliers rotate with the nut until they hit the toilet bowl, and when they do, they prevent the nut from turning, and it should loosen. You might have to spray lubricant on the bolt threads to dissolve some of the corrosion. If the corrosion is extreme enough to prevent this trick from working, you may have to drill through the nut and break it to get it off.
Installing a New Seat
After you remove the old seat, it's a good time to clean the top and underside of the bowl thoroughly with soap and water to remove mold and to disinfect with toilet bowl cleaner or vinegar. When the porcelain has dried:
• Set the new seat in position. • Feed both bolts through the appropriate holes. • Tighten each bolt finger-tight. • Continue tightening with pliers until it's almost impossible to move the seat. • Center the seat on the bowl, and then tighten the bolts all the way. • Finish up by snapping the bolt covers in place to hide the bolt heads and fitting your favorite seat cover on the seat.