ELI5:
How Matched Betting works: You are going to be using Bookmaker sites to complete the various free bet offers, but the whole idea behind match betting is that every time you "make a bet", you match that same bet on the exchange. So for example, if I bet £10 for Real Madrid to Win on the Bookie Site at odds of 2.5, I then also make a Matched bet on the Exchange (This is a separate site such as Smarkets or Betfair) where I bet for Real Madrid not to win at odds of 2.5 (or as close as I can get to those odds). In this way I am covered in all outcomes (win, lose or draw), and it allows me to fulfil the requirements of the bookies offer (For example Bet £10 and get £30 in Free bets)
What's the difference between the Bookie Site and the Exchange? On the Exchange Site you are basically being the Bookie and just like a Bookie, you have liability. If I bet £10 and my bet wins at odds of 2.5 then I win £25, so the bookies liability for this bet is £15, the extra money that they would have to give me if I win. There are calculators on the Match betting sites which you can use to calculate what Liability you need to enter on the exchange each time you make your matched bet. There is also software to help you find what games have the closest odds on both the bookies and the exchange, which is very important.
What do I do when I get my free bets? It's the same process again, You find a game that has very close odds on both the bookies and the exchange (You can do this by eye or by using odds matching software. Only this time when you use the calculator to work out your liability, you will set it to "Free bets SNR" so it knows you are not using real money. It will tell you how much Liability to use in the exchange and off you go.
How does this make me money? The fact that you have a free bet to use is what makes you money, For example a £30 free bet at odds of 5.5 in the bookies will win you £135 (30x 4.5, because the original free bet stake of £30 is not returned to you). Now let's say that the closest odds I can find in the Exchange for the same game are 6.0, I will need a liability of £112.50 to match my free bet in the bookies ( I use the calculator to work this out)
£135- £112.50 = £22.50 in Profit.
Alternatively, if my bet on the exchange wins, I will lose the free bet of £30 (but it's not actually a loss to me because It's not real money) and I will win £22.50 on the exchange. Either way, I make a Profit of £22.50
Hope this helps