How can i add customisation of text to my theme?

I have a theme with a


I want a user to be able to come and edit this theme via "Appearence -> Customize". E.g. by changing the above to

<h2> Changed </h2> 

I have tried looking at However it doesn’t go into any detail about how to change the information.

e.g. it has tautology like

"Settings automatically use WordPress's theme_mod features to get/set settings for your theme." 

Which equates to Settings change Settings???? Is there a tutorial/documentation that tells you how to create a theme customization?