We are NOT experienced enough to make a real concrete countertop. I was wondering if I could put Wonderboard on the existing countertop, use wood to make an edge all around, and then use about 1/2" of concrete over the top to make a "concrete" countertop. Would the concrete tend to crack? Would keeping it moist while drying help? Then, how do you polish the concrete? I would appreciate any advice. I have good intentions, but little experience.

Unfortunately your good intentions and little experience may lead to an unfavorable experience. You have to make a form 2" thick. I have done quite a bit of concrete over the years, but never countertops. I have seen this done on home improvement shows. You might check DIY or similar type of web-site's for more detail.

What I saw was they built a form right on top of the counter so that it was close to where it is being installed. It is going to be very heavy to move.

After building form, they used portland cement, which is the actual ingredient in concrete mix. The more portland that is used, the stronger the concrete. Mix the portland according to directions. If you are adding color, be sure to put the exact amount in each mixture. If possible, you should do the entire surface with one mix, so your color doesn't vary too much. As you are pouring it into the form, you have to "vibrate" the concrete up and down to fill air voids. Remember that the bottom will be the top when you flip it over. You will really have to vibrate a lot over on the edges, otherwise you will get ugly air voids in the concrete.

Once you have let it set up, and are ready to install, be careful not to let it drop. It will break easily, especially in the first 30 days.

Good Luck,
Al

This site may help:
Concrete Countertops - Thickness and Weight Implications for Concrete Counters - The Concrete Network