Lay betting is basically the reverse of back betting.

On Betfair:
Backing $1 on a horse paying $4 will win you $3 if the horse wins the race or you lose $1 if the horse loses the race.
Laying $1 on a horse paying $4 will lose you $3 if the horse wins the race or give you $1 if the horse loses the race.

The same thing for place bets except the horse can come in any of the 3 placings and you win/lose the same as above.

Toil, Sorry to be brutal but it might be best for you to have a look at the Betfair site and see how to lay bet as you don't want to lose everything by not knowing what you are doing.

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Never give up on a dream just because of the time it will take to accomplish it. The time will pass anyway.”

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