How do I transition from a calorie deficit for weight loss to a calorie surplus for muscle gain without gaining back fat?

I've lost about 120lb over the last two years, and I am finally at the stage where I want to transition from losing weight to building muscle.

I am planning to start hitting the gym much harder than I have been and I realize I need to eat accordingly to make the workout worth it.

However, I am terrified that once I stop eating at a deficit, I will put the wrong kind of weight back on.

I don't know enough about the science to know where to begin.

Any advice?

Weight Loss for Everyone