A Little Rusty... adding a cart to an existing site?

I am returning to a site for a client after a year, who now wants an eCart added. I am a little rusty on the procedure and have set up one from scratch but never added one to an existing site. I'd like to know best practices and workflow. One problem is that I set up tables in her database that I don't recognize for data (perhaps default eCart examples?) and the other is that as I started to set up the eCart, the wizard doesn't see the same field names as in the database but has its own.

Here is what I set up last year:
an Admin area to upload images, update the database with items, etc. in DataAssist, but I did not set up an eCart or ordering system. My catalog is hand crafted with php/mysql as is my template for the design of the system. The Admin area does not as yet have overview for orders and order lookup.
My own tables in the database are as follows:
orderdetails.
orders.
shiprates.
shiptypes.
states.

Shiprates were detailed, but I don't remember adding them and since I hadn't used the cart wizard, I'm not sure where they came from. Do they look familiar? :
2 14.75 3 1 0.55 1
3 6.8 2 1 0.8 1
4 5.65 1 2 0.75 4
5 10.5 3 2 1.7 4...

I guess my question is where to begin? Are there wizards to use to set up the entire cart at this point and admin overview? Or do I just create a separate cart function and add buy now buttons to my catalog pages? What I forget and don't see how to do now is to link the database tables above to the eCart wizard and /or modify the shipping rates and types from within eCart... Or do I need to create new tables?

I hate not using something regularly, cause you forget everything and have to start back at square 1.