Your front foot's pressure is what will initiate the carve. Backseating takes away your front foot's pressure leading to the squirrely behavior you might be experiencing.
When making the transition from heel to toe, try to focus on keeping your body weight forward more through the turn. What happens often in a heel to toe carve is you naturally want to keep your body upright "up" the hill, but this leads to backseating. By keeping your body weight forward you're actually keeping yourself perpendicular to the hill and keeping even balance over your board.
As you're shifting weight across the board and onto your toe edge, let the pressure of your front foot initiate the carve and allow the sidecut of your board to carry you through the turn.
Oh and don't forget to keep your knees bent and loose.