Load sample data after installation
If it's a new install it might be simplest to just drop/recreate the database, remove the civicrm.settings.php file, and run the installer again.

The file with the sample data is sql/civicrm_generated.mysql, but it overlaps with sql/civicrm_data.mysql. You either use one or the other. If you just import the generated file now you'd get conflicts/duplicates for things like phone types etc.