Note: it's strongly advised to use Composer. The instruction below is basically what Composer does for you.
composer.json
PSR-4
autoload section and remember it for e.g. kmit/select2
vendor/kmit/select2
vendor/composer/autoload_psr4.php
'kmit\\select2\\' => array($vendorDir . '/kmit/select2'),
vendor/yiisoft/extensions.php
:'kmit/select2'(name of extension from composer.json file of extension) =>
array (
'name' => 'kmit/select2',
'version' => '1.0.0.0',
'alias' => array (
'@vendor/kmit/select2'(path of extension folder alias) => $vendorDir . '/kmit/select2' (path of extension folder),
),
),