Here is the tutorial for creating a new module with creating a sample module - Expenses: https://community.vtiger.com/help/vtigercrm/developers/extensions/examples/entity-module.html

If you face difficulties to even after following the above guidelines, There are module builder extensions from third-parties available in the market.

The below one offer lifelong license and unlimited custom modules and unlimited custom fields:

https://www.joobilant.com/vtiger-extensions/vtiger-module-builder-relationship-builder/