David has written various books on trouser and shirt construction, teaches online sewing classes, and hosts workshops throughout the US, and abroad.
David is a friend of the site, and by all measures much better at explaining this than I am.
To make double welt pockets, you will need:
Two values determine the size of your double welt pocket. They are:
In the example below, I use a pocket width of 13cm and a welt height of 0.5cm
The welts are two rectangular strips of fabric with the following measurements:
For our example:
They will form the two narrow strips of fabric that form your double welt pocket.
Make sure to respect the grainline when cutting the two welts
The pocket interfacing is a single rectangular strip of fusible interfacing with the following measurements:
For our example:
You may have noticed that the interfacing is the same size as a welt
The interfacing is used to stabalize the pocket, and will be fused to the fabric with your iron.
The (optional) pocket facing is a single rectangular strip of fabric with the following measurements:
For our example:
You may have noticed that the facing is the same size as a welt
The facing is what you'll see when you peer through the opening of your pocket.
The facing is optional, if you do not use it, peering through the pocket will reveal whatever material the pocket bag is made from.
The shape of the pocket bag depends on the pattern. For simplicity, I will use a rectangular pocket bag with rounded corners at the bottom as you may find on the back pockets of trousers.
There is typically a longer and shorter version of the pocket bag. This is because one side is attached to the top welt, while the other is attached to the bottom welt.
Fuse the pocket interfacing to the bad side of your garment, so that it covers the place where you will make the welt pocket.
Patterns typically have notches that help you determine the placement of a welt pocket
Mark the welt pocket as follows:
The part of the horizontal lines parallel to the center line that fall in between the verticle lines is where we'll sew our welts in place. This is illustrated by the red lines.
Fold the welts parallel with the long side, with the good side out.
Don't fold them in half. Instead make one side 1cm longer than the other.
Press the fold so that it becomes a sharp crease.
By offsetting the edges, the welt edge tapers off, which reduces bulk and visibility on the outside of the garment.
In the images above, the dashed black line is the center of the welt. The red line shows where to sew, and the green line is a seam that has already been sewn.
Place the welt on the good side of your garment with the shorter folded side down, and center it horizontally.
Align the folded side with the center of the welt pocket, and now shift it twice the Welt Height (WH) outwards.
In our example with a Welt Height of 0.5cm, the fold of your welt lies 1cm from the welt pocket center.
Sew your welt in place 0.5cm (WH) from the fold, or in other words, precisely on the line you marked on your garment.
Make sure to start and stop precisely on the vertical lines that indicate the pocket width.
When you're done, sew the second welt in place. Note that this will be a bit more tricky as you'll have to fold back the first welt, and lay part of your second welt over it.
In the image, the red line marks how/where to cut open your pocket, and the green line shows the seams.
Cut open your welt pocket by cutting along the center horizontal line. Stop short of the vertical lines that mark the sides of the pocket and cut under 45 degrees to the edge of your line of stitching that holds the welts down.
Make sure to not cut into your stitches or cut too far. Make it precise.
You have been sewing your welts to the good side of the fabric. Now, we are going to flip them to the back side, through the opening we just cut in the fabric.
Flip the welts to the back, and press them flat. Take your time to work the fabric into a neat rectangle before you press it down.
Make sure to press the triangular pieces of fabric (at the short edge of your opening) to the side
You're pressing several layers of fabric. Consider pressing down (or open) the layers individually before pressing them together. This will give a sharper crease, and a neater welt pocket.
Put your garment down with the good side up, and fold it back on the side of your welt pocket, making sure to keep your welts lying flat.
Sew the triangular piece of fabric from the pocket opening to the welts.
Make sure that you do not sew into the pocket opening, as this will show the stitches on the front
Fold your garment out of the way so you can sew the pocket bag to the lower welt.
Also finish the edge of the welt with a zig-zag stitch so the welt stays flat inside the pocket.
The pocket bag is show in lining fabric here (green) far clarity. You can also make your pocket bags from other fabric.
This step is optional. Attach facing to the top pocket bag if that is want you want to show through the pocket opening.
Fold your garment out the way so you can sew the pocket bag to the top welt.
If you applied facing to your pocket bag, make sure to place it with the facing towards the welts.
Place your garment with the good side up, and fold it back at the side of the welt pocket.
Sew the triangular piece of fabric from the pocket opening to the pocket bags, much like you've sewn it to the welts earlier.
Fold back your garment at the top of the welt pocket.
Now, sew the top of the pocket opening to the pocket bag.
Now finish the pocket back by closing all the edges (the top edge is optional).
To do so, you can apply bias tape, use a serger, or sew some other finishing stitch.
After that, you're done!