I'd be glad to help you set up a discount slot in your invoice.
There are two types of discounts you can put in your invoice. One is the discount that is applicable per line item, the other is for the whole invoice. You can easily set it up on your Invoice page, here's how:
Go to the + New button and choose Invoice.
Enter the details on your invoice and select the Settings on the upper-right corner of the page.
Check the Item-wise discount if you need to put a discount per item. If you want to put a discount for the entire invoice, check the box beside Total discount. I've added a screenshot to see how it looks like.