Your question is very generic because lower back pain can have very many causes.
When I am approached to train somebody with low back pain, I always require a physician’s approval. This person should have seen a physical therapist, and it is a good idea to connect with the therapist to find out what type of exercises have been done. Sometimes the client has illustrations of exercises from the PT clinic. I then start with those exercises but still try to talk with the therapist to find out more. Be mindful that the client needs to give the okay to such a conversation.
If I get a physician’s approval but the client never went to PT, I go through a lengthy question and answer session to find out more about the history. When does the pain occur, what makes it worse, what makes it better? How long has it been?
In the event of the occasional back pain which does not appear to have any cause, I usually start with exercises lying down to minimize pressure on the lumbar spine. I encourage the client to be very interactive and always let me know if something causes discomfort. Strengthening the core with true beginner exercises like pelvic tilts and very entry level Pilates is a good starting point. As you get to know the client better your can add on from there.
Low back problems and poor posture often go hand in hand. Educating the client about posture is a key element. I firmly believe that bad posture starts initially with bad habits, and – depending on the age of the client – can be reversed to a point. Awareness is the first step in that direction.