To create an Open Item, go to the Toast back-end and select Open Items under Menus. Then Select Add to get the configuration screen below:
Menu -> Open Items -> Select Add
Once you set these configuration items up as needed, you can select it on the tablet by using the +item button.
Open Priced Menu Items:
To create and open priced item in a menu or menu group, add an item and under pricing select open price:
This will then show up on the POS as a regular item but will prompt you for price when selected: