Create a new Project on Pantheon

Install terminus: https://pantheon.io/docs/terminus/install/

Manual Setup (here complete documentation)

Enter the commands below to create a a new site on Pantheon and push a copy of this project up to it.

$ SITE="my-site"
$ terminus site:create $SITE "My Site" "Drupal 8" --org="My Team"
$ composer create-project pantheon-systems/example-drops-8-composer $SITE
$ cd $SITE
$ composer prepare-for-pantheon
$ git init
$ git add -A .
$ git commit -m "Initial commit"
$ terminus  connection:set $SITE.dev git
$ PANTHEON_REPO=$(terminus connection:info $SITE.dev --field=git_url)
$ git remote add origin $PANTHEON_REPO
$ git push --force origin master
$ terminus drush $SITE.dev -- site-install --site-name="My Drupal Site"
$ terminus dashboard:view $SITE

Replace my-site with the name that you gave your Pantheon site. Customize the parameters of the site:createandsite-installlines to suit.

Create a Swap file

If using composer you receive this error:

[ErrorException]  
proc_open(): fork failed - Cannot allocate memory

You can solve that following this approach.

Manage modules with Composer

Download modules

$ composer require drupal/module-name

Remove modules

$ composer remove drupal/module-name

On Pantheon the module are included by default into the repository, so can just commit them as usual.

If the module are not included into the repository, you will need to commit the composer files, deploy them and run:

$ composer update

Or if you are on pantheon you can use terminus

$ terminus composer <sitename>.<env> update