Set the new seat in place. Remove it from the box. The hinges on the seat will be pre-attached. Place these exactly where the hinges on the old seat were located. The box will also contain bolts and nuts. It may also include plastic caps to go over the bolts. Set this hardware to the side.
Drop the bolts into place. Take a bolt in your hand. The hinges will have holes for the bolts to be inserted into. Since the toilet seat is in place exactly where the previous seat was, these holes will also be positioned directly on top of the holes in the top of the toilet bowl. Drop the bolts through the holes in the hinges and bowl, with the head of the bolt on top. Don’t worry. Since the bolts have a head, they won’t fall all the way through the holes.
Thread the nuts onto the bolts. Pick up one of the nuts. Look at one side of the toilet bowl. You should now see the end of one of the bolts sticking down through the top of the bowl. Turn the nut onto the bolt (clockwise) until it is as tight as you can get it by hand. Use your adjustable wrench to tighten the nut just a bit further.
Repeat with the other nut on the other side of the toilet. If your hardware includes washers as well, place these on the bolts threads, then put on the nut. If the hinges have plastic caps attached (or if these are included in the box with the new toilet seat), snap these in place by pushing them down on the heads of the bolts once you’re done.
Snap the seat into place, alternatively. For some models of toilet seats, you don’t drive the bolts through the hinges. Instead, you drop the bolts directly to the holes on either side of the toilet bowl, instead of driving them through the hinges first. Then fix them in place with nuts. The seat’s hinges will snap onto or slide over the bolt heads. Double-check the instructions that came with your toilet seat if you have this variety.