On Ebay Kleinanzeigen I have bought a TP Link WR841ND Router (15,- EUR) that can be installed with the Freifunk Firmware. Download it from the Freifunk Website (pay attention to the hardware version) and flash your Router with it. The Router reboots and is available under http://192.168.1.1 (make sure your Computer is connected to the Router with a cable). You now can configure your Freifunk Network and give the baby a name. After finishing the installation hook the Router up to your network and make it boot. After a while it will show up on the Freifunk Map. Mine is called “Apunkts_Freifunk” and can be found here.
To extend the range of the network as far as possible I have disasembled the housing, bought a waterproof Lunch Box from the local supermarket, drilled 2 holes for the antennas and placed it outside my window: