A toilet seat that slips or wiggles quickly becomes annoying and uncomfortable. Outside of simply being frustrating, the situation may also lead to the need to replacing the seat altogether, if it's repaired quickly. It can also pose a danger to small children, who may climb on top of the closed seat when trying to reach objects inside the bathroom. One of the things you may need, a seat grip, is available in home improvement stores or general merchandise stores.
Examine the seat, especially the rear area where the hinge bolts are located, for any damage. If the tabs that hold the hinge bolts have broken off from the seat, replace the seat. Otherwise, unscrew and remove the nuts from the hinge bolts, underneath the bowl rim, using your hand or an adjustable wrench. Lift and remove the seat, and install a new seat of the correct size for the bowl.
Close the seat and also the lid, if there is one. Pop the covers off the hinge bolts in the rear of the seat, using a slotted screwdriver. Reach underneath the bowl rim and grip the nuts that are installed on the hinge bolts. Hold the bolts steady from above with a screwdriver while you tighten the nuts from underneath. Be sure not to overtighten. Close the hinge bolt covers. If the seat starts slipping once again, proceed to Step 3.
Remove the nuts from each of the seat's hinge bolts from underneath the rear rim of the bowl, using your hand or an adjustable wrench. Lift and remove the seat. Wipe the bowl rim surface clean where the hinge bolt holes are located, on top and underneath, using a rag.
Position the seat on the bowl, installing the hinge bolts in their respective holes. Peel the backing off the seat grip washers, and wet the washers with water. Install a washer on each hinge bolt from underneath. Steady the bolts from above, using a screwdriver, and tighten a nut onto each bolt from underneath, using your hand or an adjustable wrench.