I really like the look of the Integrity Blog Posts, like in this demo: http://demo.theme.co/integrity-1/blog/
I want each post look like in this demo; image in the top and text in bottom.
I’d like to know what I have to customize and where to do it
You can either import the Integrity 1 demo (see https://community.theme.co/kb/demo-content/) or to implement that separately, go to Appearance > Customize > Blog and under Style, choose Masonry.