Instead of focusing soley on core training, try workouts like pushups that require full body strength. The biggest key to muscle building and/or toning is diet. You must consume a sufficient amount of protein and cut back on unnecessary foods ie sugary junk- don't omit carbs from your diet, though.

First, don't run too much because this will also burn some of the muscle that you are trying to build.

Make sure that you stretch your stomach to avoid injury.

Plank and sit ups also a lot of cardio work