In Offers > My offers, choose a type of offer

Enter the name of the offer, then  "VALIDATE"

For example here, "% fixed reduction". Then follow the steps

Select the units that can be booked with this offer

Define the distributors that can book this offer

Define if there is any length of stay restriction

For example here, there is 2 nights minimum at all times on this offer

Define the booking and cancellation conditions

It can be your general conditions or specific ones 

Booking and Cancellation Conditions (GTCS) applicable

Adjust the different display criteria according to your needs

Link options created in Offers > manage extras

Apply sales limits to limit the number of reservations for this offer

If necessary, define specific taxes for this offer

Click on "SAVE MY OFFER" when you are done

Create a special offer