Require the bundle with composer:

$ composer require lernpad/repository-resolver dev-master

Enable the bundle in the kernel:

<?php
// app/AppKernel.php

public function registerBundles()
{
    $bundles = array(
        // ...
        new Lernpad\RepositoryResolverBundle\LernpadRepositoryResolverBundle(),
        // ...
    );
}

Register required repository as a service

# services.yml

app.entity.user_repository:
    class: AppBundle\Repository\UserRepository
    factory: ["@doctrine.orm.entity_manager", "getRepository"]
    arguments: [ AppBundle\Entity\User ]

Finally you need to pass UserRepository to any action in your controller:

<?php
// src/AppBundle/Controller/DefaultController.php

...
public function indexAction(Request $request, UserRepository $repository)
{
    $user = $repository->find(1);
    dump($user);
    ...