First, start with a clean dry surface.
Sand the counter by hand or with an electric sander until it feels slightly rough. When I did this it didn't look different but I could feel the texture changing.
Next, tape the edges and cover fully with primer. I used two coats of all-purpose primer. You could also use spray can primer.
Then, roll on your desired paint color. I did three coats for a very even finish but two coats would have covered just fine. The very final step (I didn't photograph) is to spray on several layers of polyurethane. This will smell very bad. I used a fan to help circulate the air.