I am glad you asked this, it's always beneficial to include parts and tools in your guides! This answer has two parts.
You can create an Item Page by clicking the dropdown menu under your login, then clicking Create New Page -> Item Page (for more detail, click here). This page allows you to include a picture and detailed information, including a link to purchase the item.
Once that page is created, it is hosted and available for use. Now when you create a guide, in the details tab, you will see options to Add Parts or Add Tools. Begin to type the new item's number and it will appear in the dropdown, then select it to append it to the document. This will show in the header material when someone views your guide.
You can also take the URL and insert it into any step as a link (for more detail, click here).