In a nutshell
If you want to place an accumulator bet, you need to use the Betfair Sportsbook.
How does it work?
The first step is to navigate to the Sportsbook app, by clicking on the icon at the bottom of the page.
Then choose your selections as you would for a single, and the betslip will appear on the right-hand side of the page and will create your multiple.
From here, you then have the option to choose your bet type - double, treble, fourfold and so forth - after which you will need to add your desired stake amount.
Your potential winnings will appear. If you are happy with your selection tick the 'Confirm Bets' box and then click on 'Place Bets'.
You will be able to access any Multiple bets placed by clicking the 'Multiples' tab in 'My Bets'.