The first step is to measure 3″ from the side in (red arrows) and 6 1/4″ from the waist edge down on the side edge (purple edge) This is going to be the pocket opening.

Connect the two points. This is going to be the pocket opening.

Next, measure 4″ down on the side (purple arrows ); this will be how deep your pocket will be and further 2″ on the waist edge (red arrows) to have as an anchor for the pocket (it gets sewn in the waist to keep it in place when worn).

To form the pocket bag square a line from the 4″ point measured on the side seam and a line from the 2″ point measured on the waist, in the picture below the red lines.

You can make the pocket bag corner a bit curved, like in the picture below. Also at this moment you can add the seam allowances. To avoid confusion I added the same as the rest of the pattern, in this case, 1/2″.

The next step is to trace the new pocket pieces. All these pieces are on the same grain as the rest of the pant leg, the pocket facing (pink paper) which you can cut out of the lining, and the pocket bag (yellow paper) which can be cut completely out of the main fabric or, in my denim pants I chose to make a small facing for this pocket bag out of the main fabric and the rest out of lining, you will see that in the next steps.

The last step to have your pocket ready to sew is to trim the new pocket opening on your front leg pattern piece like so, keeping the seam allowance.

If you are working with a thicker fabric and you don’t want your whole pocket bag made out of the fashion fabric, you can make a small facing for the pocket bag to cover only the pocket opening. You only need to be sure that it will get covered by the pocket and that your lining will not peak out.