a rough rule of thumb is that the pocket facing should be approximately half the depth of the back pocket.
The ‘welt’ of the pocket is a folded-over bar of fabric that sits just inside the bottom edge of the pocket. When the pocket is open, the back of the pocket will be seen, so you need to take this into consideration. If you want to use a contrast lining and allow this to be seen, that’s fine. But if you prefer the visible ‘back of the pocket’ to be the same as the main garment, you will need a pocket facing as well as the pocket lining. (of course, the facing could be in a completely different fabric again.)
Apply interfacing to the wrong side of the pocket opening.
Mark out the pocket placement on both the right and the wrong sides of the garment.
With the right sides together, fold the welt piece in half lengthways, and sew across the short edges, taking a 1cm seam allowance.
Trim the corners and turn the welt right side out. Use a point turner to help push out the corners. press the welt flat. Machine baste across the open side of the welt.