The concept is simple. This is what throws most people as most expect it to be difficult and are looking to over complicate it.
When you place your free bet you get to keep any winnings from it. If you lay off your free bet (which you should) you use your own money. So long as the winnings from the free bet exceed your losses on the lay you make a profit. Alternatively your lay bet wins and you haven't lost anything at the bookie as it was free money you bet with, so the lay is your profit.
This is what the calculators are for. It is important you understand how to use them. You don't need to be a mathematical whizz kid, but you do need to understand basic arithmetic. I wouldn't recommend this to anyone who doesn't have confidence with numbers as the losses if you make mistakes can be quite heavy.
"All you need to ask yourself is do you feel lucky? Well do you......Punk!"