I'd recommend studying some existing macros and spending time recreating them. Starting with simple use-cases of the macro and working up. Repeat this with a few macros and you'll eventually learn how they operate. Let Over Lambda is also a good (opinionated) book on macros (most of the book is available for free), and of course there's PG's On Lisp (jump to page 95 in the PDF if you're already comfortable with Lisp).

https://letoverlambda.com/index.cl/toc

https://paulgraham.com/onlisp.html