(1) Before you insert the new tube, carefully check inside the tire for anything that might cause it to flat again, like a sliver of glass or a thorn.
(2) Place a dollar bill on the inner wall of the tire to cover the gash. Fold it either in half or in quarters to fit the hole.
(3) Insert the tube, mount the tire, and pump it up to full pressure. The sturdy fabric of the bill will keep the tube from squeezing out of the gash long enough to get home. In a pinch, a candy wrapper or a piece of old tube will work, too.