Here’s a tip: When you’re measuring flour, start by emptying the bag of flour into a roomy container with several inches room at the top; store the flour in that container. Before measuring, use a cup measure or a whisk to fluff and stir the flour so it’s not packed down. Measure by the “sweep” method: Lightly scoop up flour in a cup measure with a flat top (not a liquid measure), sweep off top with a butter knife or palate knife. This will give you a more accurate measure.
The most accurate way to measure is to weigh flour if you have the correct by-weight measure. (Most serious bakers weigh rather than measure; this applies for delicate recipes where it really matters.)