Install the Clockwork library via Composer.
$ composer require itsgoingd/clockwork
Once Clockwork is installed, you'll need to add the Slim middleware to your app, specifying the path for the Clockwork metadata:
$app = new Slim\App(...);
$app->add(new Clockwork\Support\Slim\ClockworkMiddleware(DIR . '/storage/clockwork'));
Slim 2 is also supported:
$app = new Slim(...);
$app->add(new Clockwork\Support\Slim\Legacy\ClockworkMiddleware(DIR . '/storage/clockwork'));