payum:
    security:
      token_storage:
        AppBundleEntityPaymentToken: { doctrine: orm }
    storages:
        AppBundleEntityPayment: { doctrine: orm }
    gateways:
        paypal_express_checkout:
            factory: paypal_express_checkout

public function prepareAction() {
    $gatewayName = 'paypal_express_checkout';
    $storage = $this->get('payum')->getStorage('AcmePaymentBundleEntityPayment');
    $payment = $storage->create();
    $payment->setNumber(uniqid());
    $payment->setCurrencyCode('USD');
    $payment->setTotalAmount(5);
    $payment->setDescription('A description');
    $payment->setClientId('anId');
    $payment->setClientEmail('foo@example.com');
    $storage->update($payment);
    $captureToken = $this->get('payum')->getTokenFactory()->createCaptureToken(
        $gatewayName,
        $payment,
        'done' // the route to redirect after capture
    );

    return $this->redirect($captureToken->getTargetUrl());
  }