One of the biggest reasons I advocate you should weigh foods, as opposed to measuring their volume (when possible), is because measurements of volume are often inconsistent in comparison to weight.
One cup of the same food can produce very different values depending on the form of the food, how you pack the food into the cup, and whether the cup is level or not. A cup of diced avocado, avocado slices and a cup of mashed avocado have different nutritional profiles.
Also, cups are not uniformly manufactured. For example, a food label might say, “1/2 cup oats (45 g),” but when you actually use the cup you have at home and weigh the oats out, the scale reads 54 g or 40 g. When you look at a food label and it states “100 grams (1 cup),” be mindful that 1 cup of this food (using the measuring cup you have at home) may not be 100 g.
To ensure accuracy, weigh foods when possible.
Additionally, it’s a good idea to weigh foods in their uncooked state. The amount of time a food is cooked for affects how much moisture it retains. A food with higher water content before cooking weighs more, but it has the same nutrition profile after it is cooked when it weighs less because the water has evaporated from cooking.
The opposite is true for foods like dried pasta and rice which weight more after cooking due to an increased water content. Since you won’t always cook foods for the exact same length of time, a good way to ensure consistent measurement and tracking is to weigh foods prior to cooking.
You will also want some way of tracking the nutrients in your foods. This can be done on a digital spreadsheet, with a piece of paper and a pen, or using an integrated online food database (which is probably the easiest and most convenient). My go to app is, MyFitnessPal, but there are many others. These computer-based applications can also usually be accessed on a smartphone which is useful while traveling, going out to eat, going to a grocery store, or eating anywhere else outside of your normal element.
These applications often have information about ready-made food products as well as uncooked foods which can be helpful. Just be aware that there can be mistakes in these databases, due to their reliance on user input. So anything you consume often is worth checking in multiple places, government databases are often pretty reliable, Googling a food and “calories” will show the nutrition facts.