If you're eating garbage fast food and prepacked stuff, then it's easy. Just look it up online or read the label.

But it becomes way more difficult when you are a home cook that self-prepares meals. The easiest way is to just weigh all the ingredients in grams, then look up how many calories, say, 50 grams of butter is. It's super tedious but also very accurate. But if you start estimating, you're probably not going to correct. Calorie counting goes well with "one pot" or "skillet" meals, because once you know the total calories in the finished dish, you can just divide it into 5-8 reasonable portions to get your calorie to the 300-600 calories-per-portion range. Because it's a homogeneous dish, you can control the calories simply by weight. If I make a 5 pound casserole that I used 3200 calories of ingredients to make, then that's 640 calories per pound or 40 per ounce. Then say I wanted my meal to be 450 calories, I can just weigh out 11-12 ounces of the dish.

It's a lot of work at first, but if you record what you're doing, you can just replicate it the next time without all the work because you've already done it once.

Obviously a good quality food scale with zero-out capabilities is a must.