First, you want to finish the pocket however you plan to finish it. So if you're going to add a lining, go ahead and do that now. Here, I've serged the edges and topstitched at the top (that's the last bit of topstitching you'll see!).
Next press back your seam allowances. These press lines will be very important to you later on, so go ahead and chalk them on the inside of the pocket, too.
Right sides together, pin the bottom of the pocket only. Remember this is going to flip up when it's turned back to the right side. Stitch along the teal line, leaving the seam allowances on your other sides open so that you can fold them under.
Press that up and pin it down. Now you're going to do a zig zag basting stitch along the dotted lines. You want to catch just the edges of your pocket with the zig zag.
Now comes the tricky part. Open up your pocket from the inside, and do a regular stitch along your press line, just between the dashes that the zig zag basting stitch formed. You won't be able to go all the way down to the bottom corner, so just get as far as you can, backstitching at both ends.
Remove your basting zig zag, give it a good press, and then you can hand stitch the little openings at the corners you weren't able to grab. Voila! A pocket with almost no visible stitching!
Pretty neat, huh?