You’re on the right track that the grainline (and usually the typing) is the vertical axis of the pattern piece. Now to work out left or right:
The pocket piece is not symmetrical, so what you need to look for is the long, straight, vertical edge. This will be the edge that gets sewn into the side seam. The other vertical edge will have the curve at the bottom and not be so straight.
The long, straight side will form a right angle at the top with the waistband/yoke edge of the pocket.
That right angle is your outside, most visible bit of the pocket.
When you lay the pattern piece on the fabric (with fabric right side up) you want your little birdies looking towards that right angle, in order to be peeking out of the pocket. If the right angle is at the top LEFT of the pattern piece then you’ve got your wearer’s RIGHT hand pocket piece. Fip it over so the right angle is on the top RIGHT hand side and you’ve got your wearer’s LEFT hand pocket piece
I’d cut each pocket piece singly so you can place the fabric just right, remembering to flip the pattern piece over to cut the second one.
i hope that made sense….