Documentation for installing these two pieces of software exist but seem a bit lacking since I was not successful without some modifications I have included below.

First install the VisualEditor plugin like any other MediaWiki plugin described here under the For the General User section.

Next we need to install parsoid which is the server piece to the VisualEditor plugin. These commands are assumed you are running as root, if not add sudo.

apt install dirmngr
apt-key advanced --keyserver --recv-keys 90E9F83F22250DD7
echo "deb jessie-mediawiki main" | tee /etc/apt/sources.list.d/parsoid.list
apt install apt-transport-https
apt update && sudo apt install parsoid

Next the parsoid config, open /etc/mediawiki/parsoid/config.yaml with your favorite $EDITOR and make sure it has:

    level: info

  - module: ../src/lib/index.js
    entrypoint: apiServiceWorker
        - uri: ''
          domain: ''

For parsoid to work correctly we need a newer version of nodejs than that ships with Debian Stretch.

apt-get -t stretch-backports install nodejs

Lastly add the following to your LocalSettings.php in your MediaWiki install folder.

wfLoadExtension( 'VisualEditor' );

// Enable by default for everybody
$wgDefaultUserOptions['visualeditor-enable'] = 1;

// Optional: Set VisualEditor as the default for anonymous users
// otherwise they will have to switch to VE
// $wgDefaultUserOptions['visualeditor-editor'] = "visualeditor";

// Don't allow users to disable it
$wgHiddenPrefs[] = 'visualeditor-enable';

// OPTIONAL: Enable VisualEditor's experimental code features
//$wgDefaultUserOptions['visualeditor-enable-experimental'] = 1;

 $wgVirtualRestConfig['modules']['parsoid'] = array(
       // URL to the Parsoid instance
       // Use port 8142 if you use the Debian package
       'url' => 'http://localhost:8142',
       // Parsoid "domain", see below (optional)
       //'domain' => 'localhost',
       // Parsoid "prefix", see below (optional)
       //'prefix' => 'localhost'