Depending on when you made your booking, there are a couple of options on how to pre-book your seats. Most airlines now offer the choice to select your seat when you check in online, including easyJet and Ryanair. You will be given various payable options, including confirming leg room seats. If you do not wish to pay for a seat then one will be assigned to you automatically by the airline.
When you log into your airline's website to pre-book your seats, you will need your airline reference number which can be found on your booking confirmation email – this is not to be confused with your booking reference. Once these details have been entered, you will be taken to a seating plan where you can begin to select your desired seats.
To pre-book seats, please see this FAQ.