Which part in particular do you struggle with?
You’ll need a shadow-cljs.edn and configure your build there. Once thats done you can use shadow-cljs.edn to manage your dependencies or continue doing so via project.clj. It is fine to use lein for server-side CLJ stuff and shadow-cljs.edn for CLJS related things. I do that in all my projects but some people prefer to have everything in project.clj.
You might get a better feeling for how shadow-cljs works by following the quickstart and setting up a little dummy project.