SPREE-Integrate with existing application

Hi

I have refer this link “https://github.com/railsdog/spree” for understanding spree,

Analyzed documentation and other details from this “http://spreecommerce.com/”.

In this “https://github.com/railsdog/spree”, they have provided two different way to use the spree.I tried both the ways and got it working.

And my need/requirement is to use only the specific feature from the spree bundle.how to do it ?

Even though after referring all this, i wouldnt get the clear idea to integrate the spree with existing application.

any idea?

please provide some solution.