If it's installed via Composer then is already available in any part of your application. You didn't bother to mention which package are you talking about so I'll assume it's UnknownPackage from UnknownVendor:

// Instantiate the library
$library = new \UnknownVendor\UnknownPackage\MainLibraryClass();

// Use the library
$result = $library->someMethod('foo')->anotherMethod('bar');

But creating a service provider is a piece of cake:

<?php namespace App\Providers;

class UnknownPackageServiceprovider extends \Illuminate\Support\ServiceProvider {

	/**
	* Indicates if loading of the provider is deferred.
	*
	* @var bool
	*/
	protected $defer = false;

	/**
	* Register the service provider.
	*
	* @return void
	*/
	public function register()
	{
		// Bind 'unknown.package' shared component to the IoC container
		$this->app->singleton('unknown.package', function($app)
		{
			return new \UnknownVendor\UnknownPackage\MainLibraryClass();
		});
	}
}

And same for the facade

<?php namespace App\Facades;

class Foo extends \Illuminate\Support\Facades\Facade
{
	/**
	* Get the registered name of the component.
	*
	* @return string
	*/
	protected static function getFacadeAccessor()
	{
		return 'unknown.package';
	}
}

Note I'm assuming you are using the App namespace from Laravel 5. After adding both the service provider and the the facade to your config/app.php file (and assuming the choosed facade alias name is 'Foo') then you could use

Foo::someMethod('foo')->anotherMethod('bar');