I destroy coats. I own a wool coat and a leather jacket and I've managed to tear both right pockets and shred the linings of both. The shells of both coats are in fine shape, however. It's mending time!
Here's the old pocket. The first step was cutting it off of the coat.
Then I laid out the pocket on some canvas I had left over from the corset project. I traced out two shapes and sewed them together with a zigzag stitch, leaving the top end open, of course.
The next step was attaching the new pocket to the coat. To do this, I turned the pocket inside out and lined the edges up with pocket opening and the old seam allowances. At this point, the pocket was sticking out of the right side of the coat. I stitched the pocket to the old seam allowances then turned the pocket right-side out. The pocket now sits inside the coat, as seen below.
As you can see below, the new pocket material is invisible from the outside of the coat, just like the original pocket.
Like everything I own, the coat is covered in cat hair. That's a problem for another day.