Draw on the front the drawstring/gathering line slightly slanted.

On both front and back, reduce the same amount from each side seam at the hem for a more fitted style.

On the front only, draw some evenly spaced slash lines to open the volume to be gathered.

Cut through the slash lines and open few cm in each space (depending on the volume you want to get and on the fabric you will be using it- the softer and more drapey it is the more you can open).

Mind for the two halves to have the same final measurement as they will be stitched together.

Trace the separate pieces: center back, left front, right front.