Kiko,
It works like this.
Say you see Belarus v Scotland and Belarus are priced at 2.20 to back, 2.25 to lay. You think Scotland will get at least a point (aye, I know!) but you want the draw on your side. You decide to lay Belarus.
Now there might be £100 on the lay side asking for 2.25. You fancy taking £10 of that money. You simply hit the lay price, and it asks you how much of the money available that you want to take.
If you fill in the £10 then thats what your winning are going to be (before commission). Your liability would be £12.50.
The other option is that say the prices on
Betfair are 2.15 back and 2.25 lay. You don't actually have to lay at 2.25, you can hit the lay button, drop the odds down to 2.20 and offer to take £10 of punters money. Hit the enter button and your request to lay £10 appears on the back side at 2.20. Its just like requesting a better back price than available, where your money would appear on the opposite lay side.
If you have the confirmation option on, on
Betfair, then just have a play about with it, you will be a bit away from actually placing a bet. Hit the lay button, enter an amount you want to win, you will see your liability etc.
Betfair have a demo, I haven't used it, but I would imagine it will also go through the laying of bets. Just remember that whatever figure you enter when your going to lay is the amount your going to win, before commission.
As for bookies, well IMO, stick to
Betfair and
Betdaq. Some bookies offer prices on teams not to lose or not to win but these prices tend to be no where near what you will get for the same option on the exchanges.