Refer to the documentation on the Shopping Cart Plugin site
This plugin adds Manual Checkout to your Shopping Cart. This gateway is useful for processing check or direct debit payments. It simply authorizes every payment.
To install this plugin, just download the zip version of this repository and unzip it under /your/site/grav/user/plugins. Then, rename the folder to shoppingcart-manual.
You should now have all the plugin files under
/your/site/grav/user/plugins/shoppingcart-manual
In the Admin, go to Plugins and configure the plugin with the required information, then enable it.