I can’t teach someone design in a day, but I can elaborate on the reference and point out some of the worst habits. A lot of these designs feature ornamentation without purpose. Look critically at the content in a design it produces. Ask yourself if each piece of information is serving a purpose for the end-user. This used to be, at least a little, harder to get wrong when adding noise came at the cost of more effort. The other thing is most often just typography. For a number of reasons, text below 16px should be treated very seriously. Claude throws it around everywhere.