SPREE-Integrate with existing application

Hi

I have refer this link “GitHub - railsdog/spree: Spree is a complete open source e-commerce solution for Ruby on Rails.” for
understanding
spree,
Analyzed documentation and other details from this "
http://spreecommerce.com/".

In this “GitHub - railsdog/spree: Spree is a complete open source e-commerce solution for Ruby on Rails.”, 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.