Instructions:(cut and pasted from Monarch's website)
Use our special discount code and save £10 on return flights to the sun this weekend. It's never been easier to save, the code is available for use on all routes, all dates and all seats until Thursday!
Discount Code is only valid on new bookings, cannot be applied to existing bookings and is not valid if flights are subsequently changed. Only one Discount Code can be redeemed per booking. Discount appears as an extra fare discount in running total and total saved amount. To redeem Discount Code enter it in the box in the running total on step 2, 3 or 4 of the booking process.