You must find the ring that the power is supplied to the fan from. Isolate it by first flipping the circuit breaker, checking that no voltage is present at the connection point, then remove the circuit breaker altogether, check again that no voltage is present. Then, you should join the Live (brown in the UK) to the terminal labelled L on the fan, and the blue to the N. If the fan has any exposed metal, there will be an earth comnection on it too. If this is the case and there is one present, you must connect it. This means that if the fan is to run from a lighting ring (which it probably shouldn't, i'm not sure on this) you cannot connect it as lighting rings have no earth. As such, you must use a ring with an earth available to connect it if earth is required.
Anyhow, i'd stick with the reccomendation of getting an electrician to do this, I probably wouldn't do it myself tbh. You also need the appropriate switches etc installed.