Let’s be sure to separate goals here and be realistic.  Gaining muscle is an anabolic…losing fat is catabolic…at least if we keep it really simple, let’s look at anabolic/growth.

So, unless you’re able to pull off a really great re-comp program, you will need to let yourself slowly gain muscle, while dealing with the low accumulation of fat that comes along. 

Once you’ve gotten to a set goal, you will get to reverse gears.  Now, a caloric deficit will be implemented that will allow you to melt that fat right off.  But, you will have to toe the line to prevent muscle loss.  But that's for another post...let's stick with the basics on how to eat to gain muscle.

During the weight gain portion, your body will require sufficient calories and you will need to optimize your macronutrient ratio.  Now, that comes with various “recommendations,” here are some of those…

Keeping  this simple, here are some starting points (STARTING POINTS).  Protein should be 0.8 to 1.0 grams per pound of total bodyweight.  Fat should be between 0.3-0.5 grams per pound of bodyweight.  The rest of your calories should be budgeted intelligently utilizing carbs. 

Start your hypertrophy phase with those numbers, but be sure to monitor your weekly average change in bodyweight…you’ll likely need to adjust everything a bit as you go.  If your bodyweight does not increase, or actually decreases, add carbs until your weight starts increasing.  Your goal is to increase between 0.5-1.0 lbs per week.

Addendum: This is a basic set of guidelines and we would also have to discuss micronutrient needs, as well as address your digestion.  You can be eating perfectly, but if you're not digesting and absorbing optimally it's all a waste.