You can't really, only get quite close. You need to have a compute normal or compute terrain at the end of the line where the leaves pop sit on, with a very small patch size, less than the default meter. Maybe even 0.05m or so. You can then even try rotation, and every leaf should sit on the smallest bumps and rotate with its angle. You could try something olike this in a clean setup, just with some ground displacement and the leaves.