Push down the control knob of one of the two burners and turn it clockwise to the High position (big flame symbol), while keeping the control knob pushed down. Keeping the control knob pushed down, press the small Piezo ignitor button, which is located directly next to the control knob. When you hear a clicking sound, keep the control knob pushed down for 5 more seconds in order to deactivate the thermoelectric safety device. Check that the burner is lit by looking into the front opening of your O.F.B, keeping a minimum distance of 12 inches / 30 cm. If ignition was successful, a lightly blue aura will be under the lit burner.
This short video (German only) explains the steps to ignite the O.F.B.: