This technique is courtesy of Andrea Brown and Craftsy. Of all the methods I've tried to make double welt pockets, this one works perfectly every time. I think it's because the welts are created from a single piece of fabric so they will always sit together snugly.
CUTTING YOUR WELT POCKET PIECES
This tutorial creates one pocket with a 5″ wide opening. The pieces are cut to be 7″ wide, so you’ll be working with a 1″ allowance on either side of the pocket opening.
Cut the following:
1 Pocket lining – 7″ wide x 12″ long. Note that the pocket lining will be folded back on itself to create the pocket bag, so it should be double the desired depth of your pocket.
1 Welt (forms 2 welts) – 7″ wide x 3″ long. I
1 Pocket Facing – 7″ wide x y 3″ long. This fabric will be visible behind your welt opening when the pocket is opened, so it should match the welts.
Fusible interfacing for welts – 7″ wide x 3″ long. Fuse to the welt piece.
Fusible interfacing for garment – 7″ wide x 3″ long. Fuse to the wrong side of the garment at the pocket opening for stabilization.
1. Fuse one of the interfacing pieces to the wrong side of the welt fabric.
2. Finish one long edge of the interfaced welt piece using a serger or zigzag stitch. Also finish one edge of the un-interfaced pocket facing piece in the same manner. Pin the interfaced welt piece to the top of the pocket lining, matching raw edges on three sides. The finished edge will be the bottom edge. Pin the pocket facing piece to the bottom of the pocket lining in the same manner with the finished edge facing upward instead.
3. Sew along the finished edges to secure them to the pocket lining. Baste around the raw edges to keep the pieces in place.
4. Using a removable marker (not Frixion - they dissolve with heat!) mark the pocket opening on the wrong side of the garment. On the smooth, non-fusible side of the remaining interfacing piece, draw a horizontal line to mark its center. Draw the pocket box around this center line. It helps to use a clear ruler to ensure your lines are perfectly parallel. The box will be 5″ long, so the short edges of the box will stop 1″ from each end of the interfacing piece. The box is also 1/2″ tall, which makes the two finished welts 1/4″ each. If you want narrower welts, draw a flatter box.
5. Align the center line of the interfacing piece on top of the pocket opening on the garment. Remember your garment’s wrong side is facing up. Fuse the interfacing. Now sew all the way around this box, which marks both sides of the garment with stitches. DON'T start stitching at a corner, start on one of the long edges where you can backstitch.
6. Center the RIGHT side of the interfaced welt pocket piece on top of the pocket opening on the RIGHT side of the garment. Pin in place. Remember: Right sides together here! You don’t want your pockets to be sewn inside out.
7. Turn the garment over again so wrong side is up. From the WRONG side of the garment, stitch along the top and bottom edges of the box. Yes, everything is pinned on the right side of the garment, but you’re stitching on the wrong side because that’s where the box lines are marked. If you’re uncomfortable having your pins underneath what you’re sewing, simply move them to the wrong side, making sure the welt piece does not shift.
Use a very short stitch length at the beginning and ends of these lines to ensure you start and stop exactly at the edges. Backstitch to secure. If you notice the lines are not perfectly even in length, stitch on top of your existing stitches until they are. These lines are sewn in the darker thread below:
8. Now, mark and baste two lines that are exactly 1/4″ from the top and bottom edges of the box. These will be your folding guides to create the two welt “lips.” If you chose to make narrower welt lips, you will baste at that distance instead.
9. Turn the garment over so the right side is facing up. Fold the top of the pocket lining piece down directly along the basting stitches. Press crisply in place.
Turn the garment over again to the wrong side. Stitch along the top line of the box only, starting and stopping directly on the edges as before. This secures that folded edge in place on the right side.
10. Repeat this process for the bottom welt. On the right side of the garment, press up the bottom of the pocket bag lining along the basted lower edge.
To secure it, sew along the bottom line of the box from the wrong side. It will look like this on the right side after being sewn:
11. Fold the pocket bag lining back down out of the way. Right side of the garment still up, cut in between the two lines of stitches of the welt piece. Do NOT cut through the garment yet. You’ll just be cutting through the lining fabric and interfaced welt fabric. Cut all the way across.
12. Turn the garment over to the wrong side. Now you’ll be cutting only the garment fabric within the marked box. Cut along the center line and stop 5/8″ from the edge of the box, then snip in a “Y” shape to the corners. Get as close to the corners as you can without snipping through the stitches.
13. Remove the basting stitches above and below the box.
14. Pull the pocket bag and welts through the opening to the wrong side of the garment. Line up the folded edges of the welts, so they meet directly in the middle then press everything in place.
15. Fold the pocket bag up to match the top and side edges. Press. Hold in place while you flip the garment over to the right side. Fold back the side of the garment to reveal the pocket box triangle and side seams of the pocket bag. Pin everything in place. Repeat for other side of pocket bag.
16. Stitch down this side edge of the pocket bag, sewing directly on the existing line of stitching at the base of the snipped triangle. This secures the sides of the welt pocket opening. Continue sewing to the end of the pocket bag. Try not to catch the garment in your stitching. Repeat for other side of pocket bag.
17. Finish the pocket edges if desired. If your pocket ends up being too long for your garment, like mine was, simply sew a line along the bottom and cut or serge off the excess to shorten it.
18. Now all you have to do is secure the top edge of the pocket opening to the garment. From the right side, fold down the garment to reveal the top edge of the welt pocket. Sew along the existing line of stitching here.
Give your pocket a final press. Congrats! You’re done! Well, after you repeat all these steps for your second pocket… And you may need to wash off all those markings. Okay, now you’re done!