You must follow these steps :
Ensure that the promotion module that you want to use, is installed. All promotion modules can be found in the Modules>>Order Total section of the Admin App.
Create a new promotion in the Products>>Promotions section of the Admin App. The online help provides extra information and most attributes have floatover text with a more detailed description. When creating the promotion, you must select which promotion module to use from the Promotion Type drop list. The configuration data required, depends on the promotion type. Once the promotion has been saved, it is immediately active as long as the Active check box was ticked and the current date lies between the promotion start and end dates.
Rules may be added to the promotion by clicking on the Rules button. When a rule is selected, a pop-up window opens where the rule can be configured (e.g. Activate promotion for products belonging to a certain category). When the pop-up window is closed, the rule is saved and the Promotion Rules window shows a summary of the rules that have been set.
If the promotion needs to be activated through a coupon, you can create a coupon by clicking the Coupons button. The coupon must be given a name and a code (the text entered by the customer to activate the coupon). The coupon may also be programmed so that it can only be used a defined number of times. The final step is to enable the coupon entry field in the UI during checkout. To do this you must set the "Display Coupon Entry Field" to true in the Configuration>>My Store section of the Admin App.