Back price ALWAYS must be higher than lay price. The bigger the difference the bigger the profit. Follow the example...

Step 1: Back $100 @ 3.0
If it wins = +200
If it loses = -100

Step 2: Lay $100 @ 2.0
If it wins = -100
If it loses = +100

Combined
If it wins = 100 (200 - 100)
If it loses = 0 (-100 + 100)

So there you have a "win:break-even" situation. If the horse wins you make $100, if it loses you break even. If you want to take trading one step further you can spread that winning situation around so that you make a smaller profit on each horse. You do this by laying a little extra.

The formula to figure out how much to lay is:
AmountToLay = CurrentPossibleProfitIfHorseWins / CurrentLayOdds

So in the above situation...
AmountToLay = 100 / 2.0 = 50

Step 3: Spread the risk (lay $50 @ 2.0)
If it wins = -50
If it loses = +50


Overall traded result
If it wins = 50 (200 - 100 - 50)
if it loses = 50 (-100 + 100 + 50)

A guaranteed $50 no matter which horse wins!