How to create a table within a subtab in a Custom Record Type.

I created a Custom Record named Proforma Invoice, and added custom fields for it. My problem is with the Subtabs where I wanted to replicate what a Sales Order Entry Form has in its Items Subtab, with the intention of filling in some items that would eventually carry over to a Sales Order.

Would anyone know how to go about this? What I would want to know is how to create this “table” where the end-user could enter an Item and the other columns would just source their data based from the entry’s record. Thanks.