Note: it's strongly advised to use Composer. The instruction below is basically what Composer does for you.
PSR-4autoload section and remember it for e.g.
'kmit\\select2\\' => array($vendorDir . '/kmit/select2'),
'kmit/select2'(name of extension from composer.json file of extension) => array ( 'name' => 'kmit/select2', 'version' => '22.214.171.124', 'alias' => array ( '@vendor/kmit/select2'(path of extension folder alias) => $vendorDir . '/kmit/select2' (path of extension folder), ), ),