Definition: Feature stapling is the process by which you can add new functionalities to the existing site definition. So after feature stapling, when you create a new site using that site definition, the functionalities in the feature will be automatically available in the new site.

- You will need to create a farm solution for this implementation.

- Remember, you don’t need to change definition files (onet.xml) for this. You are just deploying one solution which will work according to the feature-scope selected.

- The feature will be activated by default only on the new sites, so you will have to manually activate the feature on old sites.

- As part of solution, you will need two features.

(1) The feature which will be activated by default or the feature with the new functionalities

(2) The feature which will be used to staple the first feature to the site definition.

You can go through step-by-step process for implementing Feature stapling in SharePoint