Separate Boxes for Post Excerpts

I’ve hunted for this through the forums and google and have hit a wall. I’m a CSS/Wordpress novice so I need help with the basic stuff.

I’ll make this as simple as possible….

My site – https://www.ramblingirish.com (My theme – Weaver 2010))
An example of what I want to achieve – https://www.backpackingmatt.com

I found this site – https://worth2read.org/2009/08/08/adding-separate-boxes-for-wordpress-excerpts/

What I need to know is how do I implement this code into my css to achieve the desired look – i.e. post excerpts in separate boxes?

I figure it’s somewhere in the weaver.css section but like I say, I’m a CSS newbie.

All help appreciated! Thank you