No mate, it doesn't matter which side wins, the result is essentially pointless.
Bet 1 - My money / Lay 1 - My Money
Bet 2 - Free Money / Lay 2 - My Money
Using this you wrote with say a bet on United to beat Liverpool.
Bet 1 - My money / Lay 1 - My Money, if United win, say the odds are 2.1 and you bet 25 quid, you would win 52.50 at the bookies, but pay out ~53 quid, gain of -50p. If Liverpool win or its a draw, your 25 you put into the bookies goes but you win the amount that you laid at betfair i.e. ~25 quid, usually works out a bit less which is where the loss comes from on betfair side. Again gain is around -50p.
So from the first pair of bets, you have lost about 50p, if you won at the bookies, take out as much as possible and put it back in betfair. If you won at betfair then you dont have to worry about anythin.
They should then credit you with your 25 pound free bet:
Bet 2 - Free Money / Lay 2 - My Money
say you bet Argentina to beat Brazil this weekend, odds of 4 and 4.2 say.
If Argentina win, at the bookies you'll get 100-25 (stake is not returned in most bookies) and at betfair, you'll pay out 75.90, loss of 90p.
If Brazil win or its a draw, you'll lose again at the bookies, but win whatever you laid at betfair i.e. around £18 for this bet at these odds.
The profit comes from the money your getting back on the second bet, the bets always counter so you always get back what you bet. But when you bet with the free bet, essentially your just obtaining the free bet again, but you can withdraw it now.
Still not clear?