How to create custom fields in customizer?
Possible Answers to Your Question:
There is no special way to do this as far as the meta handling is concerned. We do provide a framework for the form and form elements itself that you can hook into though. See the following:
If you are working on a single site install and not a theme/extension, you might find it easier to work with a plugin such as Advanced Custom Fields.
If you meant how to add custom controls in the Customizer, see the following: