The tricky parts are
1. being able to find closely matched "back" and "lay" prices on the same event; and
2. working out how much to lay.
To maximise your profit, the odds on each side must be as close as possible. So it would be trial and error - for example, take a Man Utd -v- Man City game; see what price Utd are to win with your chosen "free bet" bookie; then check what price they are NOT to win on Betfair. If the two are pretty close then you have a target.
Working out how much to lay is not intuitive. Why not? Because you have to put a figure in, and that figure is what you'll make if the selection is correctly laid. But if you lose the bet, you may have to pay out a lot more than that figure. So be careful. (Example - let's say the "lay" price is 3.5 (or 5/2 in fractional odds). You are saying Utd wil not win. If you lay £10, and they do win, then you pay out £25. So in "laying" £10 you actually gambled £25. The amount you lay just defines how much you stand to lose.
It's easier of you think of having the bet with a mate. He says, "What odds will yo give me that Utd. don't win?" You offer him 5/2. He says OK I'll bet you a tenner. You shake hands, but no money is exchanged.
If United lose, he pays you a tenner. If they don't, you pay him £25. (Or if you pretended to be a bookie, and took his tenner stake off him when the bet was struck, then you have to give him the tenner back plus his £25 total £35).
Luckily the link I provided does all this for you. For the main bookies, it searches the current odds and finds viable selections to use. It also calculates the stakes, and spells out for you what to bet with the bookie, and what to lay on Betfair. So you're spoon fed, and as long as you read and follow the instructions you literally can't go wrong.
Finally - always read the offer. It usually says your selection must be a certain minimum prce - eg 1.8 and if you pick one at shorter odds then it won't count for the offer.