I’m not going to try to recreate the wheel here–this pictorial more than adequately describes how to install a welt pocket. However, since it doesn’t include dimensions for the welts, I wanted to share how I adapted these instructions for my own item of clothing.
First, determine how long you want your welt pocket to be. It will vary, depending on the size of the piece of clothing, but it should be proportional. For my husband’s shorts (waist size 30), the pocket was 6 inches long. On the wrong side, using a ruler and fabric marker or pencil, draw a line the length of the pocket parallel to the waistband, about 3.5 inches (or what looks appropriate) down. It might look slanted, but it will look right once the pants are on a body.
Next, cut the welts: two lengths of fabric that are 1 inch wide by (pocket length + 1 inch) in length (mine were 7 inches long). To install the welts, follow the tutorial starting here, with the following variation: when sewing the welts in place, sew 1/4 inch from the edge (or directly down the middle of the welt–the distance is the same) instead of 0.5 mm (I think it was supposed to say 0.5 cm).
Next, draft the pocket. You will need two pieces. The first piece should have a width to match that of the length of your welts. The length will be your personal preference. I didn’t even measure mine–I just cut it a reasonable length. In the picture-tutorial, it shows the second piece of the pocket extending above the welt to attach at the waistband. While this isn’t necessary, it would stablize the pocket, particularly with a heavy wallet or cell phone in it. Therefore, the dimensions of the second piece are: width = (same as first pocket piece) and length = (same as first pocket piece + distance to the waistband). You want this piece to catch in your waistband seam when you sew it.
You can see in this photo how the pocket extends upward (or to the right, more accurately) to the waistband (which is the brown stripey fabric shown here).
Install the pocket as described in the instructions and you’re done! Please email me if you have any questions or need clarification: zasinzebrashop (at) gmail (dot) com.