1. After adding the products to the current sale, click on Notes at the bottom of the billing screen

2. Enter the Notes and click Save 

3. When you complete the sale the notes will be printed in your receipts

Note: You can even use the Product Modifiers. Once the product is added to the sale, it automatically asks for the modification required. Just select the modification , and it gets printed in receipts. Refer the following article to know How to add a new Product Modifier set?