Download the plugin file and unpack the content to the /callback folder.
The structure under /callback/komfortkasse should look like this now:
Open the following url in a browser: http://<your-shop>/callback/komfortkasse/install.php
The Auto Installer will start. First you will have to modify the file "store_files/<your store id>/lang/<your language>/admin/configuration.php". Just follow the installer's instructions. If you see a blank page only, edit the install.php file and uncomment lines 13 and 14 in order so see any error messages that occur.
After finishing the installer, open your admin panel and navigate to the Komfortkasse settings (or just follow the link supplied by the installer).
Make sure that "Export orders" and "Update orders" are set to True.
Choose the relevant payment type codes. Only orders with these payment type codes will be exported to Komfortkasse. You can enter multiple values seperated by comma. Normally, this will be "eustandardtransfer" and/or "moneyorder".
Choose the relevant statuses for open (unpaid) orders. Only orders with these status codes will be exported to Komfortkasse. You can enter multiple values seperated by comma. Typically, this will be 1 (open).
Choose the status that an order will be set to if the payment is received. Typically, this will be a custom "payment received" state or the standard "processing" state.
Choose the status that an order will be set to if the payment is received. Typically, this will be a custom "cancelled" state.
Do not change the other options (encryption, access code, secret keys). These values will be automatically filled in when you connect your shop with Komfortkasse.
If you don't want your shop to send out payment information (required for free and business, recommended for professional), you can use the payment module "moneyorder" and use the "Payable to" field for a description (e.g. "You will receive another e-mail with payment information." / German: "Sie erhalten ein separates E-Mail mit Zahlungsinformationen (Bankverbindung, etc.)")