Suzie, have done this for inside security pockets-sized for airline tickets as well as passport, cameras, money, etc.
1. Sew a finished size of pocket/s you need. I usually set a zipper one inch below the top. (For hip length jackets/coats, I sometimes use a diagonal zipper application. This is when the inside pocket is positioned behind the coats outside pocket.). My usual pockets have fronts and backs made of sand washed silk or med weight polyester. You can add gussets if your camera is bulky. These have worn well over the years.
2. Open the side seams of the jacket. Sometimes this is a slow process as there is a serged finish behind the sewn seam. I usually open these seams 8"-10."
3. Position the finished pocket with safety pins. Try on the jacket with your item in the pocket-and note if location is suitable. If an outer pocket usually carries glasses, put them in for the trial.
4. Pin the pocket to the lining. You might find a small wood or plastic sheet useful when pinning. Baste in place.
5. Carefully stitch pocket in place around outside edges.
6. I usually serge the lining edges and then pin them together so the serged edges are inside and topstitch the opening closed.

Some of the travel purses are similar to what I sew. Good luck-once you have these, you will never want to be without!

Thank you.
How would I insert an inside breast pocket? This is just a casual jacket - zip front, etc.
That's why I could only think of the way I described above...
I like the idea of adding gussets...I'll have to figure that part out, too. I was thinking maybe velcro or encased elastic at the top opening to keep things from sliding out when/if he lays the jacket down. That actually happened with his other jacket - but, luckily we retrieved what fell out.
Thanks again.

I think the only way is to hand sew it. You could make a regular pocket, or you could put a zipper, velcro or closure of your choosing and line the pocket & hand sew it on.