A ripped tire is bad news. It doesn't perform well, can't be pumped up fully, and the tube bulges out through the rip, just waiting to pop. By applying a tire boot, you can ride on the tire. This will get you down the trail, and if the rip isn't too big, it may even let you ride the tire until the tread wears off.
But I wouldn't recommend jumping off ledges with a booted tire. Repair the tire and salvage your ride. But first chance you get, buy a new one. Put the repaired tire on your kid's bike, or give it to your neighbor who only rides pavement.
Also lightly sand the outside, being careful not to expose the cords. If there are raised letters in the area, smooth them off so the outside patch can cover them.
Wait until the cement is dry -- both inside, outside, and in the exposed cords.
Reapply a very thin layer of rubber cement to the outside, then wait until it's dry to touch.
Hint 2: Wrap a strip of inner tube around-and-around the partially-inflated new tube at the place where the sidewall is ripped, before putting the tube back in the tire casing.
Hint 3: For large tears, or for long rim cuts (such as this 4-incher on Matt's rear wheel), put cable ties around both rim and wheel. This keeps the expanding tube from pushing the repair open.
Hint 4: A dollar bill, folded in half, is strong enough to keep a tube from bulging through a cut in the tire.