This is a pocket often used as the back pocket on mens' trousers although of course you can add one to whatever garment you wish... in this case I have used for the breast pocket on a mens' tailored shirt.
Measure and mark the position of the welt pocket. My welt pocket measures 13cm x 1.5 cm (5 1/8" x 5/8"). Cut two strips of fabric equal in length to the length of the welt pocket plus two seam allowances, and equal in width to twice the height of the double welt; in this case my strips measure 15cm x 3cm. Also cut a strip of fabric on the bias for the loop closure.
Stitch the bias strip, and cut the stitching threads leaving a long length, at lease as long as the bias strip itself. Thread the long end through a needle, and with blunt end leading, pass it through inside the strip. Turn the strip right side out.
Press the welt strips in half lengthwise, wrong sides together, and place pins in each to mark the length and width of each welt. In this case, a length of 13cm and a width of 7.5mm.
With the pressed folds of the welt strips facing out and the open raw edges towards the middle; put these pins in exactly the same pinholes as those previously made to mark the welt position. Arrange the welts to lie smooth, flat and straight.
Stitch the long edges of the welt, in an exactly 13cm long line, positioned exactly 7.5mm in from the folded (outer) edge of the welt. Note; the folded edge is the visible and therefore the important one here; the accuracy and precision of your stitching MUST be relative to that outer edge).
Fold to snip into the centre of the welt. and cut to about 1cm in from the start and finish of the stitching. From this point, snip outwards in an inverted arrow shape into the very corners of the welt.
Turn welts to the inside and pin the loop closure in position to the seam allowance of the upper welt, at the centrepoint. Stitch.
Pin the pocket piece to the lower welt seam allowance, and stitch exactly over the previous stitching. Press down.
Stitch a button through both thickness of fabric; the shirt layer and the pocket layer, for stability, using the loop closure to determine the position of the button.
Fold up the pocket piece and pin to the seam allowance of the upper welt. Stitch exactly over the previous stitching and press.
Complete the box of stitching around the welt by catching down the little triangles at the outer edges. Continue stitching down the sides seams of the pocket.
Finish raw edges by overlocking or zig-zagging. You can also finish by binding with bias binding; I tried doing this for the pocket of one shirt, the purple pin-stripe pictured below. It looks real nice but it made the pocket feel kinda bulky and stiff. I would probably just stick with the overlocker in future.