What you need to know
Flexbox distributes items along one primary axis, which makes it useful for nav bars, button groups, and content rows. Grid controls rows and columns together, so it is often clearer for page regions and card galleries.
How to apply it
Ask whether the layout is mainly a row or column, or whether both dimensions matter. Try Flexbox for alignment within a component and Grid for a defined collection of tracks; combine them when that keeps the CSS simple.
Common mistake to avoid
Do not force one layout tool onto every problem. A simple block flow is often better than introducing flex or grid for a single element.