Start with no more than 30 feet (9 metres) of line and work from there. Get a good, controlled casting cycle underway, with controlled loops. Make sure your stroke length is manageable and then adjust inclination so that your fly lands just on or immediately above the ground/water. Then, shorten the line by three feet, tighten the stroke, and adjust inclination to get the fly to again land just on or immediately above the ground/water. The fly should be landing in a target area that is now closer to you. Then, shorten the line by three feet … get it?
Keep shortening the line length, tightening up the stroke, and adjust the inclination until you’re down to a range of no more than 20 feet from your position. If it helps, visualize the backcast and forward cast being exactly opposite one another as you get shorter/tighter/steeper/closer. That’s one half of the Trajector-cize. Here’s the other half:
Start shorter/tighter/steeper/closer and work your way back out to longer/stretched/shallower/farther. Add three feet of line to each casting cycle and watch for the fly to land in a target area that is farther away from you each time. As before, it may help to visualize the backcast and forward cast being exactly opposite to one another as you go. Once you’re back to where it all began, you’ve completed the basic Trajector-cize. Want to be sure it’s all good? Video yourself.
For many casters, the closest target (like 20 feet) can be harder to hit accurately than a target at nearly twice that range. The trouble comes from trying to use the same backstroke length and position as for a target at say, 45 feet. That immediately changes the trajectory of the whole cast, and usually results in a forward stroke that is not only too long, but also arcing downward as the caster attempts to force the fly to the target. Kiss good aim goodbye with that.
Being able to control trajectory better really can make a difference. More precise fly delivery will only help in tougher angling situations.