-DONT CUT THE FUEL PIPE!!- because i did lol (the fuel pipe comes from the same area as the vacuum pipe)
Find the brake servo under the bonnet on the drivers side (big round thing, might have 'bosch' written on it or something) there should be a vacuum hose connected- its the only hose that comes out of the brake servo (about 10mm in diameter) follow the hose round to the engine and fit the valve as close to the engine as possible. Both ends of the pipe might need to prised apart to get the ends of the valve on (this takes a bit of time)