It's near impossible to "teach" a volley with written word, but here goes...
On both FH and BH you need to lay the racquet back and let your upper torso "do the work". You're "chopping" the racquet at the ball and down towards the court, I assume to create backspin. With a racquet face angled less than 45 degrees to the court, slice the racquet under the ball by using the pace of the ball... don't swing. After that, able to "cup" the ball control the volley... then try to add some punch by rotating the shoulders and torso.