You mean one that has been removed from the rim? Apply tire lube to the tire bead seats, work the tire onto the rim, and then inflate the tire. The tire will pop onto the rim. Don't get your fingers in there, it will hurt.
The best thing you can do to reseal a tire is to take the tire off the rim and use a good rubber cement and a heavy duty rubber patch. Then you can put the tire back on the rim and shoot it with a cheetah while the air line is on it. Then install a new valve core.
