How to go from a flat stomach, to some nice abs in 3-4 monthes before bikini season?

What should I do to get a nice firm and toned stomach and abdominal muscles before its beach weather and time to wear a bikini again?

I need specific exercises to follow or anything that will help.
Please do not suggest diet pills or any of that.

Can anyone help?
Also, school softball starts in about a month so that’ll help.