Expanding Shopping Cart Tutorial

I'm following the Agile Development with Rails shopping cart tutorial,
and while some of its sticking, I'm still far from proficient.
Basically, I'm trying to use their shopping cart example as the base
for my store, but I need products in the store to have line items
themselves- for example:

my site sells custom printed t-shirts wholesale. I'm imagining a
customer will add a t-shirt order to their cart, and then from there,
be able to add line items - 5 smalls, 5 mediums, etc. Anyone have any
general suggestions on how I should set the tables up and associate the
line items with the products? So that a customer could have multiple
shirt orders, each with it's own set of line items?

Many thanks in advance, and I'm happy to trade printing for code.

You might be interested in my book about building ecommerce sites using Rails: