Hi Dave,
Basically you take the air filter housing off, place a spacer(which has the gas pipe connected to it)and runs to a gas regulator which in turn connects to your gas supply.then put the air filter housing and filter back on using longer bolts(which they supply). then disconnect the fuel solonoid and run the feed wire to centre spade on the three way switch(which they supply also), then connect a wire from the switch on either top or bottom spade back to the solonoid, the spade you have left run to the gas solonoid on the regulator(make sure the regulator has a good earth) that way you can switch between gas or petrol as you require. The centre of the switch will be off position, and the other 2 positions will either be gas or petrol depending on where you choose to connect the gas and petrol solonoids. I just make sure I don't run out of gas though as I'm only paying 55p a litre now.
Hope this helps
John