The neutral grip is mainly used in between shots, as it helps you change quickly to other grips. It can also be used for hitting certain shots.
Start with a forehand grip, and then change the angle slightly towards backhand. This is only a small change in angle, less than 45 degrees.
The pad of the thumb stays off the racket. It should not be placed directly onto the racket.
Don’t curl the thumb. Instead, keep it straight. This will help you twist the racket handle in your fingers, making it easier to change grip.
Some details for you to try
You may want to hold the racket slightly more out towards the fingertips than for other grips. This may help you change grip more easily. However, this is only a small change; the racket should not be held all the way out in the fingertips!
Similary, I find the neutral grip works best when the index finger is relatively high. Most of your ability to change the racket angle comes from the index finger and thumb working together. Try to find a position where you can use them effectively.