Open the radiator filler (at the front of the engine bay) so you can keep topping it up with water. Top it up as far as you can get it but don't put the top back on.
At the back of the engine bay on the left hand side (near the carb) should be a pipe with a plastic screw in it.
Get the engine going, then lift the screw point on the pipe as high as it will go, then undo the screw. This will pump the water round the system, and the air bubbles caught in the system will naturally rise to the highest point (where the screw is undone) and come out of the system.
Make sure you are filling the reservoir with more water/coolant/anti freeze (a mixture thereof) as the level drops.
It might make sence to have an extra pair of hands on site to top up the water level and rev the engine a little to help you out.
(at least, that's how I did it. Others might improve on the method!)