CodeIgniter 3, how to configure the .htacces to remove the index.php

I'm a little dizzy about this and I'm not good at this kind of configuration. I have a project as follows:

ProyectoPrincipal/
      Backend/
        application/
        system/
      Frontend/
        application/
        system/

That is, both the back-end and the front-end are subprojects of codeigniter . I would like to know how to set .htaccess to remove index.php . I tried many solutions but it only works in the back but not in the front, it just returns a 404 .