First of all, a merchant has to sign up for Google merchant center account. The merchant's e-commerce store must comply with all the technical guidelines given by Google to successfully display a badge on the store. Check all technical guideline here.
Read this answeres about code too. These answeres will help you to understand settings of plugin (ex. OPT_IN_STYLE)
You will be able to install and configure plugin very easy.
If GCR badge is not showing