It takes time and much more than most people are willing to wait for results.
How long Does it Take
My Perspective:
The main rules are to be patient and consistent and focus on an area at least at first.
I learned this by gaining more core strength but not actually attaining it through the standard approach. I was doing curls with my back braced against a wall and sitting on the floor. This exercise was to strengthen my arms to prevent injury. As I progressed on this I began to notice that not only were my arm muscles getting larger and stronger but that my stomach muscles had started showing signs of definition that I hadn't seen in decades. I kept at it and added other items like a medicine ball to work the same magic.
Diet
My own experience with this suggests that the weight lifters and body builders really know the answers when it comes to adding muscle. I started taking protein supplements in conjunction with weight training and began to see a rapid visual improvement in my tone and muscle size in my arms and upper body which is where I had focused my effort.
All of these things are just my own experience so not being an expert on this subject I suggest that you do your own research. All I've done iis to tell you where you might begin your research.
This post is for informational purposes only and does not constitute advice on how to train for strength or muscle enhancement. Do you own research please.