First, download cornerstone.zip file from your Envato account.
Then login to your WordPress backend, head over to Plugins -> Add New
Then click on Upload Plugin button, and upload cornerstone.zip file to install it.
With regard to Cornerstone related tutorials, please check the heading "Cornerstone / Content Builder" on our knowledge base section (https://theme.co/apex/forum/t/knowledge-base-overview/287).
Hope that helps.