Dry vs Liquid Measurements—What's the Difference?
No matter how many times I Marie Kondo my kitchen, I will never ever get rid of one kitchen tool: my liquid measuring cups. They're bulky and take up a lot more space than nesting measuring cups, but they're essential to precise liquid measurements. That's because dry and liquid ingredients are measured differently—liquids by fluid ounces, which measure volume, and dry ingredients by ounces, which measure weight.
How to measure dry ingredients
In a perfect world, you'd use a scale to measure dry ingredients, but those stacking dry measuring cups with handles that you definitely have in your kitchen will do just fine. You can measure a precise amount by filling the cup (or 1/3 cup, or 1/4 cup, and so on) with the ingredient—like flour—and level it off with a knife or other flat tool.
Flour and white sugar should not be pressed or packed down, but recipes usually call for a "packed cup" of things like brown sugar. This means pushing the sugar down to tightly pack the measuring tool with the ingredient and no air.
There is no way to know exactly how many dry ounces are in a cup, because it varies by weight—use however much fits in the cup measure you're using, or use a kitchen scale to be more precise (advisable for baking).
If you tried to measure flour by volume, you could really mess up a batch of cookies—1 cup of all-purpose flour weighs 4.5 ounces rather than 8 ounces. (There are 8 fluid ounces in 1 cup by volume for liquids.)
How to measure liquid ingredients
Measuring liquid ingredients is really simple: have separate measuring cups with liquid measurements. There are 8 ounces of liquid (fluid ounces) in a cup, so two cups is 16 ounces (aka 1 pint), four cups is 32 oz. (aka 1 quart), and so on. They're measured in liquid measuring cups—made of glass or plastic with a handle—and can be poured easily from its spout.