Install pandoc:

sudo dnf install -y pandoc

Ensure that markdown is enabled in init.el:

(doom! :lang markdown)   ; <-- make sure this line is present

Add the following to the end of config.el:

(after! markdown
  (setq markdown-command
        '("pandoc" "--from=markdown" "--to=html5" "--standalone")))

Run doom sync and restart Emacs.

Bring up the markdown editor with the following key combination:

C-c C-c l (or <localleader> p) to preview

Updated: