Navigate to Admin -> Promotions -> Shopping Cart Price Rule, from there fill in the fields as needed. When you get to the "Action" tab:
- Apply: Percent of Product Price Discount
- Discount Amount: The % off you want (just the number), e.g. 50
- Apply to Shipping Amount: Yes (this targets the shipping)
Now you want to make sure you don't target any products with the discount, so continue on the "Action" tab in the "Apply the rule only to cart items matching the following conditions" section, create a rule that applies to none of your products (e.g. SKU is Blank).