The overhead cast is the most basic fly cast. Learn to execute it well and you will be able to easily adapt the skill to the side cast and backhand casting.
Good casting starts with learning to grip a fly rod correctly and adopting the right stance to maintain comfort and balance.
Gripping the Fly Rod
The normal grip is with the thumb on top and slightly to the left of center (assuming a right-handed grip) so that the ‘V’ between the thumb and the index finger is in line with the top of the rod. Your grip should feel comfortable and firm - but not tight. Your rod and reel only weigh a few ounces, so it won't require a death grip to contol it.
If you prefer, placing the thumb directly on top of the rod is acceptable, and you might find this useful if extra force is needed on the forward cast. Another variation is sometimes used for accuracy when casting short distances, or just as a “change up" to relieve fatigue during a long day of fishing. Instead of placing the thumb on top of the rod, try shifting the index finger around so that it lies along the top of the rod instead.
Fly Casting Stance
The correct stance is important to maintain comfort and balance. It's very easy to lose your balance when trying to get the most distance out of your cast, or to lose your footing on the loose, water polished rocks in the bed of a stream.
The proper fly casting stance is to lead with the foot on the same side as your casting arm (i. e. right foot forward for a right-handed caster). Your feet should be set approximately at shoulder width for balance and stability. This will allow you to easily transfer body weight from one foot to the other during the cast.
Start the cast with the fly rod extended horizontally in front of you with your forearm and the rod in a straight line, and the line straight. Accelerate smoothly in an upward direction making sure that the rod tip stops just short of vertical (the “12 o'clock" postion) so that the line will project backwards above the horizontal plane. In other words, the line will still be rising as it continues backward. If you go beyond vertical before stopping the back cast, the line will go downwards! A precise stop causes the rod energy is to be transferred to the line, and catapults it through the air.
Once you have stopped the back cast, pause so that the line reaches full extension above and behind you. Once you begin the forward cast, accelerate the the rod forward smoothly and stop the forward movement when the rod is at approximately the “10 o'clock" position. The line will project forward and straighten as it falls towards the water. Follow through with the rod to ensure that it lands taut, straight and softly.
Note that the vertical plane has been used for this discussion. That's why this method is called the overhead cast. The line flies overhead and over the rod tip. The same can be done in any plane to make straight line casts. Once you have mastered the overhead cast, you can apply the same techniques using the horizontal plane to keep the line low and avoid obstacles; or cross your body on the back cast, bringing the right hand toward the left shoulder in an off-vertical plane for a backhand cast.
More Fly Fishing Tips, Techniques, and Resources available from My Secret Stream.