The grip when hitting a one-handed backhand should be held so that the “v” formed by your thumb and index finger is on the first bevel to the left of the top of the grip( for a right hander).
From the ready position, turn your shoulders and hips, placing your right foot (for right hander) in front of the left with your knees slightly bent. The racquet head should stay slightly above your wrist and hand until just before the point of contact.
Let the racquet head drop as it moves towards the ball. Try to contact the ball at or in front of your right foot (for right hander). Follow through with the racquet high and pointing towards your target. Work on hitting your backhand mainly in a crosscourt direction initially, to encourage an early hit. Emphasize keeping your shoulders sideways, with head and eyes focused on the ball until after the point of contact. Try to repeat the same swing on all of your shots by using your feet to get the ball in a similar hitting position.
Watch Federer, Thiem, Wawrinka, Dimitrov, and Gasquet to get a good visual of the basics of a one-handed backhand.