We currently do not have a step by step guide on how to create a contact page but it should be straight forward to do it using Cornerstone.
Please check this link on how to build a page using Cornerstone.
Also, if you wish to add a contact for on your site, Contact Form 7 is a bundled plugin with X.
Please check this link on how to setup the form using Contact Form 7:
Once you have created the form, you can get back to Cornerstone and use the Contact Form 7 element to add it on the page.
Hope this helps.