How do I do it?
Type in the barcodes on the site and it gives an instant cash price, though it asks you to trade in a minimum 10 at a time. If you accept the offer, just follow the on-screen instructions and it gives you freepost labels for posting.
Since it doesn't accept responsibility for non-delivery, it could be worth sending it – depending on the value – by "Recorded Signed For" delivery which insures it for up to £46 and costs 77p on top of usual postage.

You then get paid by cheque upon receipt (items aren't returned if they fail basic quality checks though). It aims to send the cheque within seven days though feedback from forumites suggests it can be much longer.
How much?
It depends on the title; as a rough guideline you'll generally get up to about £3 for DVDs, £2 for CDs, and £6 for Blu-rays and computer games, though it can be a fair bit less.
Can I do better elsewhere?
Yes. Though hoarders of 90s CD relics could make £100s on the side, for more modern items you may be better off selling 'em individually on eBay* or other auction sites. You may do better trading in computer games at high street game shops; check these too.