Using the index finger of your fretting hand is most likely the biggest challenge you’ll face when forming barre chord shapes. As mentioned, the finger should be just slightly above the fret. Even still, you may struggle to get the strings to ring out cleanly. To ensure that all strings are pressed against the fretboard with your index finger, use the harder side of the finger rather than the softer part on the side of your palm. One way to do this is by pressing down your index finger and then “rolling over” so that the bony side of the finger presses the strings down.