How do you make the discount a percent of sale instead of a dollar amount? Entering the percent symbol after the number does not work.
In the WordPress admin, if you go to WooCommerce > Coupons, and press Add New you'll see a Coupon Data meta box (or edit an already existing coupon).
Add a Coupon Code if one doesn't exist.
In the box on the General tab there is a Discount type setting dropdown. To make the coupon a percent of sale change it to "Cart % Discount", then enter an integer between 1 and 100 in the Coupon Amount field.
Press Publish and you should be all set!