Summarize this article:

The back of your toilet seat is usually connected to the toilet with two long screws that run through the porcelain at the back of the bowl. The screws are secured by two nuts underneath. In the front of the cover, look for a small groove that is designed to help you pry up the cover. Then, use a small screwdriver to pry up both the seat and the cover. In cheap toilets, the screws are made of plastic. Most toilets, however, use steel screws, and more expensive models may even use brass or stainless steel. Be especially careful with plastic screws! If the seat is loose, then it may wobble back and forth, in and out of alignment with the rim of the toilet bowl. Straighten out the seat so that it rests evenly atop the bowl. Consider sitting down to test it out for comfort. Turn the screwdriver clockwise to tighten. Remember: "righty tighty, lefty loosey." There's usually a nut with a wing attached to prevent the nut from turning. If there isn't,  hold the nut with a rag while you tighten the bolt. Decide which screwdriver is the best fit. Make sure that the screwdriver head is big enough to fit into the notches of the bolt head. If the screwdriver is too small, then the bolt will not turn. The friction from the slipping screwdriver will quickly wear down the bolt and make it unusable. If the bolt keeps turning without tightening up, clamp the nut underneath with a small pair of pliers. Clamp onto the end of the nut, and hold it steady as you screw the bolt. Once you have twisted the bolt several times, the wing on the nut should prevent it from turning anymore. To loosen the nut, spray it with WD 40 and wait 5 minutes. When the bolt feels tight, give it one more quarter rotation so it's extra secure. Once the seat does not wobble back and forth, push down the lid. It should click closed.

Summary:
Find the screws and pry up the cover. Center the toilet seat on the toilet. Tighten the bolt. Apply tension. Keep screwing until the seat is tight.