Although the fender grab is somewhat outdated, it still is a good base trick to learn and practice because it uses the fundamental skills of shifting your body weight while letting go of the bars.
When you are approaching the jump, be sure to gain the majority of your speed on the face of the jump itself. Many entry-level riders tend to approach the jump at high speed and blurp the throttle over the face. This will keep your bike strait and level, but for a fender grab (or any trick where your bike will need to pitch upwards) you want to slow down a bit and accelerate on the face. Be careful no grab too much throttle or you may loop out. When you leave the ground, your bike will be automatically pitched upward. Just before bringing your hands off the bars, pull the handlebars up to your chest slightly. Lift your hands off the bars so that they are high enough to clear your levers. Then reach forward and grab the fender. Some people like to grab the fender right by the number plate and others like to grab the fender at the end. This is a matter of preference and comfort. Only hold your hand (or hands) there for a second before you let go and reach back for the bars. You will notice that by leaning over the front of the bike that the upward pitch you initiated by pulling up and accelerating on the face of the jump will be countered by hanging over the handlebars. When you reach back for the grips, the bike should be approximately level.