To be fair to the OP, it is a legit method of making a few quid, but its far from risk-free.
I made my first cash online like this, and its all about the maths. But there are FAR better resources for learning the techinque, and with the risks and work involved, and the fact that once you have exhausted LEGIT bookie sign-up offers you're done. You're looking for any free bet offer and angles to exploit it.
The idea is to loose at the bookie, so you transfer the free bet amount (minus what you've lost in the spread) into Betfair.
Most use this as start-up cash (like I did) to get into Betfair Trading. Look for the Geeks Toy or BetAngel Forums if you're interested in learning about it, but its TRADING, and like any other trading, it carries risks. You're looking to take advantage of the price movements of the betting markets or before the match/race starts or 'in-play' (betting while the match is being played) To put is simply, you're betting on one player/team to win or loose, and if they get ahead/go behind, you bet on the other outcome, locking in guaranteed profit (or loss if it goes against you) on the difference in prices.
LOL rather google it, much better explanations out there.
You can make good money trading sports/horses, but again, theres a ceiling, google the Betfair Premium Charge. Its hard to beat this unless you have a massive edge. Its why I've moved away from it, although I still trade for fun sometimes.
AND like other trading, MOST people will fail.
I'd focus on IM as theres bigger potential and the opportunity to build a sustainable long-term income. Betfair can (and has done - premium charge) change the rules on you whenever they please. Its one of the reasons I've moved in IM.
But as Micky said, that pdf & site is all about the affiliate links...