How can I apply discount percentage to one order line and not the whole order?

Here is how to do it, apply a 0% discount to start with which will apply to all the lines on the order. Once you've set 0% you can't change the discount on those lines. Now key in the item or items you want to discount then discount it by typing the amount of discount followed by the % button. You'll end up with an order with some items at 0%, and others at the value you set.

See illustration below showing two items with 0% discount, one with 15%, one with 99% and another at 15%.

Mark