Start with a photograph, preferably one that you took or one you have permission to use. It should be medium to high resolution. Place the photo in your Illustrator document (File > Place). It doesn't matter if it's linked or embedded. Make a copy of the photo and place it off to the side or above the original. We'll use this later for reference.
With the first photo selected, go to Object > Create Gradient Mesh, which will transform this photo into a vector mesh. In the dialog box that appears, enter a fairly high number of both Rows and Columns. Try to make it proportionate to the aspect ratio of your photo. This is the grid that we'll be using to build our clouds, and it's going to take a lot of detail to achieve a realistic final image. Choose Flat as the appearance and keep the Highlight at 100%.
