Calories

I started to workout and eat healthier more often once quarantine started then May came along and I was on and off. Now I am getting back to workout and eating healthy. But I tend to eat a lot of calories one day not enough the next. I’m usually missing close to 800 calories or more in a day but I honestly don’t know how to stick to eating exactly the amount needed so then I tend to snack to add calories. Any suggestions on meal prep or anything?