The higher odds you choose, the more likely your first bet will lose and so you will receive the free bet! Note this will require a highly liability in Betfair:
|£30 Trigger Bet Back Odds
||% Chance of Losing (desirable)
To make this even easier and faster, I compile the best bets with high odds available each day. Check out the blog for the most recent 'Trigger Bet and Free Bet: Weekly Selections' and scroll down to the free bets section.
To view decimal odds rather than fractional odds, use the top right menu: