When you get your install pack you will have a "filter" maybe two! plug this into your wall socket and leave it in, connect your phone to the filter outlet, leave that plugged in also.
You will be better with a 10M modem cable rather than a phone extension, then you can go direct from your modem to the [ADSL] socket on the Filter. As for disconnecting the modem lead when not in use, I can't see any problems with that, others will advise if I'm wrong though. j.