The easiest way to create a custom role is to customise an existing role. Then you can add/remove permissions to tailor the role to your specific needs.
Navigate to Setup > Users/Roles > Manage Roles > Customise an existing role. Eg. CFO role.
1. Give your new role a name.
2. Set the relevant permissions for your new role, add/remove permissions as required.
3. Click Save.
You can now assign your new role to employees.