Download the plugin file and unzip the contents of the folder /copy_this to your OXID installation. In OXID 6+, copy the module directly to the /modules folder.
Open OXID eShop Admin.
Select Extensions -> Modules.
Select Komfortkasse from the list.
Open the Settings tab.
Make sure the two checkboxes are checked. ("Export orders" is needed for orders to be transmitted from your shop to the Komfortkasse system. "Update orders" is needed for updating the payment date back to your shop.)
Enter the payment methods you want to manage with Komfortkasse. (You may enter multiple payment methods separated by comma.)
You can manage prepayment, invoice and COD orders. If you don't want to manage all three payment methods, just delete the fields that you don't want to manage and leave the respective field(s) empty.
Don't touch "Advanced Settings" until told by Komfortkasse Support.
Open the Overview tab and hit Activate.
That's it! Now open a Komfortkasse account and connect your shop.