Perhaps the major thing to understand about the long irons is that you must have confidence in your ability to use them. If you shudder each time you reach into the bag for a long iron, you are using the wrong club. You should use a fairway wood instead, or even play safe with a middle iron.
Developing confidence is a question of mastering the fundamentals and then working on them in practice. A smart way to prepare for a practice session is to be thoroughly loose and warmed up before hitting the first shot. I will usually take out my two-and three-irons and swing both of them together until my muscles feel stretched and comfortable. Then I will hit a few short-iron shots and work through the middle irons before getting down to serious effort with the long irons.
The thing to think about with the first practice shots is just meeting the ball solidly. Never mind where it goes. Then start to pick out targets and hit toward them. All the time, meanwhile, you should be concentrating especially hard on keeping the head steady.
Once you are out on the course, a few further ideas about long irons might provide some comfort. First, do not feel you should hit all of your long-iron shots onto the green. (In last year's U.S. Open only 65% of the irons hit to a 168-yard par-3 hole ended up on the green.) The middle-handicap player should be happy to get somewhere around the green. Second, remember that while the club face looks frighteningly narrow it actually supplies more hitting area than a fairway wood. Third, the shafts of these clubs are long, thus giving you a wide arc and plenty of club-head speed. It may not feel that way, but your club head is traveling plenty fast, so there is no need to rush the swing with your hands. Fourth, the shaft's extra length allows you to take a more upright stance than you are able to with a short iron. This gives you better balance. And, finally, when you begin to master the long-iron shot, you should experiment a little. You may find yourself able to do things with long irons you only dreamed about in the clubhouse. But never try the impossible. Do not use long irons when you have difficult sidehill lies or treacherous trouble shots that are hard enough to play with a short iron. If you want to see how tricky long-iron shots can be, come out to a PGA tournament sometime and watch us touring pros butcher them.
Unless you are an exceptionally skilled player, do not use a long iron out of any lie that is not good, even in the fairway. If the ball is sitting well down in grass or clover, a wood or a middle iron should be used. Also forget about the difficult finesse shots, such as trying to hit high long irons. Only a golfer who plays in the 70s can carry these shots off with any consistency.
But, above all, don't be discouraged. If you can recognize the limitations of long irons while still appreciating their potential, you can start enjoying the use of clubs that too many players have come to hate.