I haven't taken any photos of the process so far, but I will try to describe two ways to do it.
Just words- bear with me.
For a jacket breast pocket or a waistcoat pocket.
Take a piece of patterned fabric (stripes or plaid) and draw an angled line on the wrong side of the fabric to represent the sewing line of a pocket welt. Mark the beginning and end of the welt too.
You need to see this line from the right side of the fabric so line baste this marking by hand.
You will need a rectangular piece of fabric for your welt pocket. It should be big enough to cover the area of your pocket from about an inch below, an inch and a half to each side and three to four inches above.
Take a sharpened piece of clay tailors chalk and using a ruler, chalk the sewing line on the right side of the fabric. Chalk a vertical line at the beginning and end of the pocket too.
Take your rectangle of fabric, with the right side up and carefully lower it on top of the chalked lines, matching the patterns in the fabric as you put it in place. Make sure that you have a larger area of fabric above the line- you only need a small amount below the line.
Pat gently to transfer the chalk from the front of the garment to the wrong side of the rectangle.
Lift away and you should now have a chalk line to use as your sewing line.
Triple check visually and then line baste. Trim the excess fabric below the line as this will be your seam allowance.
Baste it to the garment right sides together along the line and triple check that when you turn the welt up, the patterns in the fabric continue to match both along the sewing line, as well as horizontally.
Make your welt pocket.
This is where I like to use a fusible.
Cut a straight grain strip of woven fusible the height of your finished welt, for instance, if your welt is 1"tall by 4 1/2" wide, cut a 1" straight grain strip about 5" in length.
You need a rectangle of fabric for the welt as described above.
Take your garment fabric to an ironing table. Lay it on the table with the good or right side of it facing up. You can see your basting which indicates the size and position of the welt.
Lay the fusible sticky side up in the position of your welt. Trim the ends of the fusible to the finished shape of the welt.
Now take your welt rectangle and position it over the fusible, lowering it carefully, as you match the patterns in the fabric. Triple check that you have not accidentally shifted the position of the fusible.
Take your iron and press gently, so that the fusible sticks to the rectangle of fabric over it.
Lift the rectangle off, and your welt shape is now in position.
Trim the fabric below the welt to your desired seam allowance, and check again before making your welt pocket.
I'm sure that there are other ways but I confess to being a fan of the second method, and we all use it with success. It gives you the finished shape so you have the angle of the sides as well as the top fold line. You can use it for flaps too.
If you use the first method, only use a clay chalk that will brush off your garment and leave no permanent marks.