Begin with a basic pick up and lay down cast—there should be no slack in the line before you begin the back cast motion. This will allow for an efficient transfer of power through the rod tip to the line. The line should extend straight out from the rod tip.
Then, pull and lift the line with your wrist firm but not necessarily locked. This movement will incorporate your acceleration to a stop. Your hand should move in a straight path during this acceleration—the path your hand takes is the path the rod tip will take.
Load the rod for reach cast. Force the rod to flex, storing energy that is transferred to the line with the stop. At the end of this hand movement, make a firm stop at a high point so the line goes upward and not toward the ground. This firm stop will allow the rod to unload its energy to the fly line and create a loop.
Next, pause to allow the line to straighten completely. This will provide the forward cast with all the energy to transfer into the rod in that direction. This pause is a very essential part of your cast—without a correct pause your line will fall behind you or you will begin forward too early. Both of these will cause loss of energy in the line.
Make a habit of watching your back cast. Seeing when the line straightens will improve your timing and help create a good technique. A good back cast helps create a good forward cast.
Practice should always have a direct purpose. Aimless practice can easily add bad habits to your cast. A bad habit learned and practiced can be difficult to overcome, which why I recommend starting with lessons.
Proper practice will quickly build confidence in the basic pick up and lay down cast, so you are confident when you walk up to the water with the desire to get the fly to the fish on the first cast.