You have to use a quick and fine setting concrete like Portland cement or Quickrete. Also, consider making the "pieces" in sections so they are not so heavy to lift and put in place. They can be "combined" after they are set in place with a light spread of more concrete to connect them in looks. Here is a tutorial to help: https://www.hometalk.com/26850157/concrete-countertop
Here's several ways you can do it Debbie! You don't mention if you want to start from scratch or go over existing countertops so I have Included both! Looks like a daunting DIY project but one that is doable if you do a lot of research and take your time!
This is a good video tutorial: https://m.youtube.com/watch?v=9lRaNI2AOEQ
I've used Feather finish twice and love the results. https://www.alifeunfolding.com/blog-1/2019/5/28/how-to-diy-a-concrete-bar-top-in-a-weekend