Login to Google Merchant account and follow the steps below.

Go to promotions [1] > Lists [2] and click on (+) [3]

Add a Promotion ID [4]. Make it something you can recognize later. Select the language [5]. Then describe the promotion [6] and set the effective dates [7].

Now go to the Feed rules-tab. Select attribute promotion id [10].

Now you can create rules which will set the promotion ID to your selected products. For example all product from brand X wil or all protucts op type Y. Make sure the promotion ID is in the same case as in step [4].