I can't mention enough that concrete is heavy and you'll need people to help you with the final installation. I was able to move all of my countertops with 3 people, but that's because I did my biggest piece in 2 sections. Make sure you have a battle plan of how you'll move it and set it down.
The weight of the countertops will keep it in place if you just apply a generous amount of caulk on your plywood top. If you'd like added support or live in an area where earthquakes are prevalent, you may decide to drill underneath into the concrete and use tapcons to secure it further.
I also wanted to hide the 3/4" plywood top that supported the countertop, so I bought 3/4" by 4' strips of galvanized steel at Lowe's. I just cut them down to size and glued them onto the plywood edges using Liquid Nails. It adds another decorative touch and looks great!
If you remember, I had that aluminum strip that I wanted to be an accent where the 2 island pieces came together. To do this I cut the aluminum down to the proper length allowing it to stick up (and out) just slightly from being flush. I applied caulk to both sides of the aluminum and pressed the countertop together, sandwiching the aluminum and holding it in place. I then used 2 part concrete epoxy, which I spread over the aluminum and onto the concrete. After it dried, I ground it down with the wet polisher until it was perfectly flush with the 50 grit, then worked back up to 1500 grit to match the finish.
The Cheng Pro-Formula kit came with a penetrating sealer that I applied according to the directions which were simple. I chose to add extra shine and protection by polishing them with carnuba wax. The wax is heat resistant, so you can still put hot pans on the countertop without it burning, and gives added stain protection from acidic foods like wine and citrus. The wax needs to be re-applied about once a month in order to keep up the shine and protection.
So now I've got beautiful new countertops that I made myself! It was a lot of work and a fair amount of expense, but it was so much cheaper than granite and I was able to add details that you can't find in any other solid surface countertops. I'm going to enjoy my fruits of labor for now, but eventually I'd like to do an outdoor kitchen which will need countertops as well. I'll document that process as well when I get around to doing it.
I hope this helps you with your project and I'll be more than happy to answer any questions you have, so feel free to ask. Major thanks go to Mr. P and my brother who helped me with this project, I couldn't have done it without them.