Go to your accountant portal, open the Integrations submenu under Settings, and click Available in the DATEV tile. On the next screen, click Connect. You will then asked to log in with your DATEV credentials.
Note: only the client files that were added in DATEV at the time of establishing the connection can be linked with Billit. If new files are added in DATEV, you need to log in again by removing and re-establishing the connection. Files that are already linked will remain linked.