An easy way to reach your desired weight, whether you are trying to gain it or lose it, is to align your food intake with your weight goal. All you need for this is a scale, to know where you are at and where you want to be. Second, you need to figure out how many calories you currently need to maintain and how much your goal weight would need to maintain.
Imagine person A is 100 pounds and eats 1000 calories to maintain that weight. They want to be 110 pounds which is a weight that requires 1300 calories to be maintained. They eat an extra half chicken breast plus half an avocado daily (which amounts to roughly 300 cals) and slowly but healthily, they reach their goal weight.
Yep, it's as simple as that. The main barrier we face is the fact that we usually eat pretty sporadically, missing meals here and there or over eating on certain occasions.
To keep it simple, remember the following.
Record what you eat
At least for the the first couple weeks, just to make sure you are eating the exact amount of food you need. You only have a count your total calories once to set your daily goal, and then softwares like my fitness pal can help you track your food easily.
Even though tracking food can be boring, it's the best way to reach you goals.
Set smaller goals
I recommend breaking it down in goals of 10 pounds at a time. This will make the changes in your diet sustainable. Yea you could create a huge calorie deficit to lose weight quicker, but you will also lose muscle. And if your calorie surplus is too big when trying to add weight, you will be packing more than just lean mass after just a couple weeks.
Plus if you drastically change your diet over night, your body might not respond very well and may cause you to be sick or lethargic and your energy level will take a hit.
This is going to be a gradual process. In general, the going rate of increasing lean body mass is 2 pounds per month and for losing weight it's 4 pounds per month.
You can go quicker but then it's not as safe and it is a little bit trickier. Here's a funny story for you: for 2 months late last year I dieted hard for a show. It was awful but well worth it. The problem was that after the event I was dieting for ended, I basically consumed everything in sight and gained 20 pounds in 4 days. I went from 2500 calories a day to almost 5000 calories a day. Let me assure you it wasn't 20 pounds of lean muscle mass. That is called the metabolic rebound.
I also see the reverse where people tell me they are doing a weight loss diet and then I see them a few days later and they are completely exhausted and can't even exercise!
So patience is key if you want to experience healthy and quality changes.
Adjust macros accordingly
However you are modifying your diet, keep it balanced! If you are going to lose weight, don't just think "oh I'll just drop all my carbs or all my fats." Likewise, eating more protein alone wont help you gain all the muscle you want.
A well balanced diet is the best way to keep sustainable results. If you need to decrease you calories by 300, take away 100 calories from each carbs, protein and fats.
What about cheat meals?
You may have to do additional calorie counting if you want cheat meals. It's completely ok to have cravings or want a cheat (you should here me fantasize about pizza), but you need to have a basic understanding of what you are putting in your body. Cheat meals are usually not very filling and pack a ton of calories, so you might need to adjust your other meals to make room for the deliciousness you may want to indulge in.
For exemple, a medium pepperoni pizza has roughly 880 calories, 28g of fats, 44g of of protein and 112g of carbs. For most of us that would mean eat basically no carbs for the rest of the day.