Hey guys

I am 5'9" and about 160 lbs. I am satisfied with the shape that my body is. I dont want anything more or any less. However, I just want to gain more muscle, especially in the abdominal area (I gotta fit into my pants lol). The way my body is right now, I like it and now, I know where exactly to work on (arms, chest, abdominal). I dont want to look like a body builder nor do I expect or want to be like Scott Herman.

I know diet and exercise is the key. But exactly, how do I go about this?