How do I create offers "Buy X Get Y 10% off" in Free Shipping & Hello Bar app?

In this app you can create informative messages of sales at your store. If you want to actually run the sale, like Buy X Get Y % off, you can do it using Upsell For Products - Buy X Get Y app:

In one window, select the target products from your catalog after purchasing which a customer will see the pop-up with e.g. "Buy these relevant products for 10% off!"
In the other window, select the product with the reduced price from your catalog (you’ll need to create these products with reduced price in advance). The customer will see in that final offer popup when they click checkout.
In this app (Free Shipping & Hello Bar), create the informative message so that the customer knows about this sale.