To use a Free Play, just check-mark the small "FP" box on the right side of the Bet Slip, next to the "Risk / Win" fields. You'll know the Free Play is being used when you see the risk amount shown under "Risk FP".
If the amount you enter exceeds what you have, a message in red will tell you what your current available Free Play total is.
You do not need to use your entire Free Play amount on one wager. You are welcome to split this up into several smaller bets if you want.