To put the tyre back on, place one side of the tyre into the rim. Next, inflate your inner tube to approximately 30% before putting it into the tyre (inflating it beforehand reduces the risk of pinching the inner tube).
Then, insert the other side of the tyre into the rim, starting on the side opposite the valve. Before inflating the inner tube all the way, check to make sure it isn't pinched between the tyre and the rim.