Here is a short lesson from Jason Borgers web site.
Learn a Mend
Casting is the core physical skill of fly fishing, but in order to angle effectively, you also need to know how to mend. Mends are primarily meant to combat drag, which is movement of the fly (usually unnatural) due to currents pulling against the line and leader. There are two types of mends: in-the-air mends and on-the-water mends. In-the-air mends are created after the cast has been made (after the “stop” of the rod on the forward cast), but before the line has fallen to the water’s surface. On-the-water mends are created after the cast has been made, and after the line has fallen to water’s surface. We'll look at two nearly identical in-the-air mends here: the Reach Right Mend and the Reach Left Mend.
These mends are primarily for presenting a fly across a stretch of current—usually across faster water onto slower water. Such a fly presentation situation can be tough even for experienced anglers, but the Reach Right and Reach Left can help anyone cope. These mends are not difficult to make, but do require some practice to hone.
To make a Reach Right, make a standard Overhead Cast. Once you have stopped the rod on the forward cast and the loop is unrolling in front of you, smoothy reach your arm (or just allow it to tip over) to the right, stopping when it reaches the horizontal plane (left-handed casters will have reach across the front of their bodies). This makes the line fall to the right. Now, make another Overhead Cast followed by a reach to the left (right-handed casters will have reach across the front of their bodies). This is the Reach Left and makes the line fall to the left. Regardless of direction, make sure that you are smooth as you reach. Also keep in mind that the fly will be pulled back toward you unless you “shoot” some line as you reach.
Shooting line is allowing line to slide out through the rod guides after you make the cast, but before the line falls the water’s surface. Try shooting immediately after you stop the rod on the forward cast—while the line is still actively unrolling through the air—for best results.
Once you have got the basic reaches down, it is time to apply them. Here is a real-world example: You are fishing across a river, with current flowing from right to left, casting a mayfly dun over faster mid-stream water onto slower bank-side holding water. If you cast straight across stream, the line closest to you will get pulled down-current the fastest while the end of the line, then leader and then the fly will drift at continually slower speeds, respectively. It won’t take long for your fly to shift into speedboat mode and make a very nice “V”-wake as it drags away.
To avoid the problem, aim the cast toward the target, and as the line is unrolling in the air, reach to the right and shoot line simultaneously. The fly will drop just up-current of the target, with the line coming in at a down-and-across angle. As the line drifts down-current, follow it with the rod tip (moving down-current and toward your left). This following move is very important, and I often stay slightly ahead of the drifting line so that I have an additional “safety cushion” against drag. The extra line on the water (the “slack”) will help to reduce the effects of drag on the fly. Of course, it won’t last forever, and once your run out of slack, drag will be right back with you (but at least you may get a critical extra second or two of undisturbed float over the fish).
