Casual Symbol Overlay includes the base Casual package as a dependency. Casual has the command casual-init which will setup all modules within it as specified in the hook variable casual-init-hook. Casual Symbol Overlay can be included in this hook with the following Elisp added to your Emacs initialization file:
(add-hook 'casual-init-hook #'casual-symbol-overlay-init)
Note that this will only work if Casual Symbol Overlay is installed via package-install.