Hey,
was looking for the right composer command to install a particular (old) demo-site for comparison. Maybe obvious, but for others helpful: (in install documentation off topic and here google will find easier as in slack)
for Neos version 2.3.9 in subfolder neos-demo-239 use:
composer.phar create-project neos/neos-base-distribution neos-demo-239 2.3.9