How do I get yarn installed on elastic beanstalk?

You can customize packages that are installed and commands that are run on deploy with .ebextensions

For yarn, I created a file with the following commands which install a recent node version and yarn:

# .ebextensions/yarn.config
commands:
  01_install_node:
    command: |
      sudo curl --silent --location https://rpm.nodesource.com/setup_8.x | sudo bash -
      sudo yum -y install nodejs

  02_install_yarn:
    # don't run the command if yarn is already installed (file /usr/bin/yarn exists)
    test: '[ ! -f /usr/bin/yarn ] && echo "Yarn not found, installing..."'
    command: |
      sudo wget https://dl.yarnpkg.com/rpm/yarn.repo -O /etc/yum.repos.d/yarn.repo
      sudo yum -y install yarn

More Documentation: https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/customize-containers-ec2.html