A soldering iron or heat gun is the best heat source for shrinking heat shrink tubing. You may also use a lighter, but be careful not to overheat the tubing.
Apply heat evenly around the tube until it shrinks to the desired size. If you're using a heat gun, keep the gun moving so you don't overheat any area. You should also hold the tubing away from your body and wear protective gloves. Once the heat shrink tubing has cooled, it will be tight and secure.