In the root's repository on your local computer, run:
composer install (the first time you create the zip)
vendor\bin\robo build
You will find the package in /dist. Make sure you have the php zip extension enabled!
Install the package pkg_migratetojoomla.zip (soon, you will be able to install regular releases. Right now, go to Code -> Download zip. Unzip and use pkg_migratetojoomla.zip in your Joomla installer).
Go to System -> Manage -> Plugins -> filter by migratetojoomla and enable all plugins.
This tool only works for WordPress at the moment, where Joomla and Wordpress are installed on the same server.
Go to Components -> Migrate to Joomla.