The manual covers it pretty well, but you will want to top it up as full as posible, then run it to get hot with fluid in the resivoir to draw in, then when cool, open and top up resivoir, then get it hot again and crack the bleeder pugs to release air (all this while heater is on 'full blast').
Once cool again, top up resivoir again. That should do it.
If the heat is poor, repeat. | 
Linear Mode
