If you don't know the answer to that, then you haven't got a
Betfair account, which makes it a bit difficult to explain.
You can limit what you could lose on a lay, exactly the same as you can limit what you could lose on a back.
If your bank is £200 and you want to lay a 2/1 shot for half of your bank, you simply enter the odds of 3, and enter a stake of £50.
If the horse is above 2/1 and you don't want to lay higher, tick the 'keep' box on the screen. When the race goes off the lay will not cancel, and if it hits 2/1, you could lay it.
Be aware that money matched is done so on the order it's put in the system. If there are 5,000 people who got their lay in before you, if and when the horse hits 2/1, they will be matched before you.