If Casual is installed using package-install, then you can use casual-init to setup support for Eshell.
By default, casual-init will setup support for Eshell via the hook function casual-eshell-init which is included in the customizable variable casual-init-hook.
Use the command customize-variable on casual-init-hook to control inclusion of casual-eshell-init via a checkbox interface.
Refer to the Simplified Install (Install) section for more detail on customizing casual-init.
Casual makes opinionated decisions on the keybindings used in its menus. Such opinions are extended from said menus to the keymap(s) of a particular mode. For Eshell, this can be controlled via the customizable variable casual-eshell-add-extra-keybindings (default t).
Users who want a more granular install of Casual modules can invoke the hook function of interest in their Emacs initialization file.
(casual-eshell-init)
For users who have manually installed Casual outside of package-install, the following guidance is offered.
The primary interface for Casual Eshell is the Transient menu casual-eshell-tmenu. It is autoloaded ((elisp)Autoload) so if Casual is installed via package-install ((emacs)Package Installation) then casual-eshell-tmenu can be invoked via execute-extended-command (M-x) without need for modifying your Emacs initialization file.
That said, Casual is designed with the intent of using a common (key)binding across multiple modes to lower cognitive load. This document uses the binding C-o for this purpose, but can be changed to user preference.
There are many ways to configure this binding. Shown below is a straightforward implementation:
(require 'casual-eshell) ; load all dependencies including `eshell' (keymap-set eshell-mode-map "C-o" #'casual-eshell-tmenu)
If lazy loading of the eshell package is desired then try using eval-after-load:
(eval-after-load 'eshell (keymap-set eshell-mode-map "C-o" #'casual-eshell-tmenu))