BOMs and Routings can be entered in item's details.
1. Go to the Stock section.
2. Find the item.
3. Open its details and scroll down to Bills of Materials and Routings
4. Click on the "+" button to add new.
Important! If an article is defined as a procured item (option "This is a procured item" is checked), then these sections are hidden and Purchase Terms table is displayed instead.
Read more about stock items here, BOMs here, and Routings here in the User Manual.
BOMs and Routings can also be imported in bulk: see here and here in the User Manual.