Then click on "My Account/Mon Compte": and login using your credentials:
Enter your username and password.
The login is your phone number followed by "@freeadsl".
The password is the one that was provided by Free (If you haven't change it of course)
Once you are on the cofiguration page/interface hover your mouse over "Ma Freebox" (My Freebox). Then click "Configurer mon réseau Wifi/Configure my wireless network"
Via the same page:
Check the "Activer/Enable" to activate Wi-Fi.
Give a name to your network
Choose your type of protection
Create your network key
Validate your configuration by clicking the "Sauvegarder les paramètres/Save Settings"
Rebooting the Freebox
Restart your Freebox (either by unplugging-plugging the power cord, or by holding down the "?" button of the Freebox remote and choosing the appropriate options. If you want to connect multiple computers to your Freebox wifi you should also enable the router mode of the Freebox.