Its probably on another topic in here somewhere, but in essence, you must stop the actuator on the drivers side exhaust. There are two ways of doing this, either by blocking the vacuum line to the actuator on the exhaust, or, by removing the plug to the solenoid which resides inside one of the roof mechanism covers in the boot of the car.
I have this modification and have done it by blocking the vacuum line. Look under the car, look for an S shaped piece of rubber tubing a few inches long, coming from the back of the car and finishing on the exhaust flap vacuum actuator, carefully remove this pipe. Insert a small object that can be removed if required, such as a golf T, hence the name. Put the pipe back into position, enjoy a louder exhaust.
There are numerous topics online about this, simply do a google search. You may hear many people stating do not do this because it may generate codes, or damage catalytic convertors, or breach emissions etc etc. These are simply not true, the flap is solely to quieten the car at low flow rates, preventing droning or loud start up sounds. It opens at high flow rates to prevent restriction of exhaust gasses.