Just some few lines, what you'll do for each of your site :
• A file to paste at the root of the site (https://github.com/PrestaShop/PrestaShop-webservice-lib/blob/master/PSWebServiceLibrary.php);
• Another one to paste in "classes" directory ;
• Another one to paste in "override/classes/webservice" directory ;
• Some specific rights will be given the the "output" dir of the module (777) ;
• Some grants for your orders assitants, in order they can access the admin tab of the module ;
• Install a specific webservice on each site to query.