Very cool example.
Only by custom code
and some AJAX for page transitions. In your example i think this is custom code but maybe try barba.js:
http://barbajs.org/demo/grid/index.html
How to? by searching for webflow freeancer with Javascript
background -or- by learning barba.js (forum Q about this):
Anyway, no way to answer “how to do this” by forum Q.