I begin by washing the rock and letting it dry well. The next step is to start filling in the base to create a standing platfrom. This is a trial and error process. It usually takes a couple of days because I have found it is better to build a little at a time versus a lot of thickness at once. I use a pre-mixed concrete patch. After much experimenting with various brands, Ace hardware brand is what I prefer.
Once you have a well balanced rock, I then take very rough grit sandpaper and sand down the cement and entire rock including any imperfections I may have filled in. I don't try for a perfectly smooth surface. It is after all, a rock. Once sanded, I give a base coat of black paint, let dry and do a rough outline of the design. I start with the base colors which will take a couple of coats. I am not too worried about perfection at this time. On the next coat I will start tightening up the lines before I start laying in details.
Before you begin detailing, you have to decide what style you will paint. I prefer what I call a detailed folk art style versus and accurate depiction however I have done both. This is where you have to know the taste of the person who will be receiving.