What are the steps should we follow at development time to make our application modularized?

I am working on a product, We have many different modules in our application.

What I want is to design my application such that I can give any of the package to my clients as per their requirement.

Eg: I have seven modules 1-7, there is a possibility that for one client only one module is good enough and for some other 3-4 module is good. Then I just need to install only those modules on their server.

How should I take modularize approach?

I will be using WEBApi and REST service.