The first thing that you need to is to practice your form. Try using an online guide, such as the ones found on YouTube, before you hit the court to have an idea about an ideal shooting form. To do a proper free throw, move your arms to form an “L”, make use of your knees, flick your wrist just as your release the ball while giving it a backspin, and complete the motion with your follow through.
Once you have learned the correct form, practice it by hitting the court and starting off close to the basket. Train your form before you start hitting the free throws. Make the shots just as you make them from the free throw line. As you successfully make the shots from close to the basket, move back a step and shoot from there. Continue this routine until you reach the free throw line. Now start shooting from there. Keep doing so for an hour. Maintain your proper shooting form throughout the course of the practice.
Film yourself while you are shooting from the free throw line. After you are done taking the shots, take a look at the video to examine what you may have been doing wrong. Look closely at your posture, the movement of your arms, your knees, and your wrist as you flick the ball. If you notice something that needs to be corrected, focus on it during your next practice session.
Try getting a shooting aid. It will help you in correcting your form, while forcing you to flick your wrist and follow through while shooting. There are a number of shooting aids available in the market, but you should consider getting yourself a ShotLoc, as it has been scientifically proven to help with the form.
All your efforts are likely to be in vain unless you commit to practice. Return to the court everyday to practice your free throws. If you do not have time to practice every day, trying hitting the court at least four days a week and that too for longer practice sessions.