Yii is great, but it lacks so many tutorials and so little help.
All people ever give is some extension to cart session. That’s not what I’m looking for! I’m looking for how to do it, but not someone who wrote it and then I can’t fit it into my project.
Ive been looking for MONTHS!! There’s nothing good.
Well, my vote is for shoppingcart extension:
http://yiiext.github.io/extensions/shopping-cart-component/readme.en.html
But personally I’ve wrote e-commerce suite from scratch recently. It’s the best way in my opinion - you get exactly what you want, nothing else.
And your rant about lack of documentation makes all that great people who contribute to yiiframework ‘by example’ section upset 
I can recommend you Larry Ulman book to start with. It was a good starting point for me.
For months?..
Shopping cart can be created from scratch in a couple of days 