Replacing a toilet seat in most cases is an easy task that just a few very basic hand tools. The toilet seats come in either a standard round or, elongated shape that fits almost all toilets made.
Most toilets use the standard round or, elongated shaped seats and have bolts that go down through the toilet seat hinge, through the toilet, and have the protruding bolts and nuts visible on the underside of the toilet bowl. The measurements on a standard toilet seat is for the hinge bolts to be 5½” apart and a round seat will extend about 16½” to the front from the mounting holes, and an elongated seat will extend about 18½” to the front from the mounting holes. If this is the case with your toilet you are among the many that have an easy job ahead of them.
Some toilets may have unique shaped seats or, fasteners that that go into a blind hole without access to the underneath. In these cases you should investigate the availability of seats and how they are secured before going any further. You should always investigate when blind fasteners are used, when the standard replacement seat hinges do not appear to line up properly with the existing seat hinges, and when the shape of the existing seat is unique not matching the replacement seat. Some proprietary replacement seats can be very difficult to locate and you may have trouble getting one quickly.
First clean the area under where the hinges were, there is almost always some staining and dirt there that couldn’t be cleaned while the seat was installed. Some of the replacement seats use adhesive pads under the hinges to help keep the seat secure, follow the manufacturers instructions for cleaning and attaching adhesive pads. Place the bolts through the hinge if you have a replacement seat that has separate bolts, then slip the bolts through the seat mounting holes in the toilet and finger tighten the nuts in place. Check the placement of the seat to make sure it is properly aligned the tighten the bolts as needed from above and below as needed taking care not to over tighten and strip plastic bolts, or crack the bowl if metal fasteners are used.