When you are matched betting, the amount of money you make from each free bet depends on the odds you have used for the qualifying bet and the free bet.
As a general rule you will make the most money if your first bet is low odds. For instance backing a horse at 1.5 and laying it as 1.7. However make sure you read the terms and conditions carefully to ensure you have qualified for the free bet.
For your free bet, you will want to bet on high odds, at least above 4 and preferably higher if you have enough liability in the exchange.
Use the odds searcher to find the best events to place your bets on and filter the results with the column headers.