Instructions
Select fabric for the shirt pocket. This is easiest if your pocket is going to add a decorative element: choose a fabric that complements, contrasts or brightens up the shirt's original material. If you want the shirt pocket to match the shirt, the best place to take fabric from will be the shirt tail (or lower half, for t-shirts); ensure your shirt has sufficient fabric to do so.
Cut the pocket from your chosen material; exact dimensions will vary, based on the shirt's size and how conservative or flamboyant you want the pocket to be. Square pockets should be cut to the desired size, plus 1/2-inch on each side and 2 inches at the top for a seam allowance. V-bottomed pockets should be cut in a rectangle with the vertical measurement being the height from the center of the v to the top of the pocket, with the same seam allowances added.
Iron the pocket to create finished edges. With the pocket piece lying right side down, fold each edge in, press, fold again, and press. For example, on the left side of the pocket, fold the pocket in 1/4-inch; press; fold again to create a clean edge and press. At the top of the pocket, create a finished edge of 1-inch. Create a V-bottomed pocket by folding and pressing at the desired angle.
Sew a straight seam along the top of the pocket to ensure the top finishing remains flat. (This edge will not be sewn when attaching the pocket to the shirt.)
Pin the pocket to the shirt front, ensuring the ironed edged remain flat and unrolled. Position the pocket as desired: centered on a dress shirt's left panel, at an angle or even at the hip.
Carefully sew the pocket to the shirt along the left, bottom, and right edges, using a thread that complements the pocket fabric.