Change gallery format to look like the demo

Hi

I’m trying to create my first blog, so am very new to this. I hope somebody can easily answer my questions ??

I’ve managed to create a gallery to display a number of pictures, but they are in a simple 3 wide and square format.

The Sela demo (https://selademo.wordpress.com/blog/) under Blog -> Weekend recap has a gallery with the pictures set in different sizes and positions and sat closer together. Does anybody know how to set the gallery to this format?

Any help appreciated, thanks.