I want to add a sub menu item \"Locations\" to \"Catalog\" menu item in opencart administration. On selecting locations, I want to see my own location management view page which
How to create a opencart admin module??
You can simply do this by adjusting:
Admin > controller > view > template > common > header.tpl
You can simply make adjustments to the menu on this page (static changes). To actually create modules for you and your staff etc. Then follow the tutorial of MVC posted on this page:
How to create a custom Admin Page in Opencart?