The EntityManager is the central access point for the ORM functionality. Bootstrapping it is easy:

use DoctrineDBALTypesType;
use DoctrineORMEntityManager;
use DoctrineORMTools;

Type::addType(
    'post_id',
    'InfrastructurePersistenceDoctrineTypesPostIdType'
);

Type::addType(
    'body',
    'InfrastructurePersistenceDoctrineTypesBodyType'
);

$entityManager = EntityManager::create(
    [
        'driver' => 'pdo_sqlite',
        'path'=> __DIR__ . '/db.sqlite',
    ],
    ToolsSetup::createXMLMetadataConfiguration(
         ['/Path/To/Infrastructure/Persistence/Doctrine/Mapping'],
         $devMode = true
    )
);

Remember to configure it according to your needs and setup.