Let's start with £10. You place a bet on some event with odds around 2.00. If you lose, you place a £20 bet on another event with odds around 2.00. If you lose again, you will place a £40 bet on another event with odds around 2.00.
Now, if you win this bet, you will win £40 * 2.00 = £80 and your sum of stakes was £40 + £20 + £10 = £70. So you made a profit of £10 or your profit is equal with your first stake.
That is what a simple martingale progression strategy is about. It is great, isn't it? The only problem is that stakes can go high rapidly, so if you go into a losing streak, you can lose all of your money easily. If you lose 5 bets in a row, your 6th bet should be staking £320 which is 32 times bigger than your initial bet.
So, Martingale would be great system if we have unlimmited amount of money in our account, and if our bookmaker have no limits for bets on some event. Unfortunately, that is not the case in real life, so you have to be careful if you are trying to implement a similar system.
Since the problem is that stakes go high rapidly, you can try to decrease the progression by choosing events with higher odds like HT/FT X/X for example, or 1:1 correct score. If your chosen odds are higher, then you don't need to double your stake every time you bet and you can calculate which stake will still bring you the profit. Of course, since odds are higher your losing streak could be longer.