I start some of my more complex illustrations with a pencil—the old school way. I also still work on a drawing table with push-pins stuck in precise spots in the corners of my drawing table to establish vanishing points. Once I establish that, I get the drawing going with the proper perspective. Many times I pick and chose what details I draw initially in pencil and which ones I put in directly on the computer. After creating the pencil rough, I place in Illustrator as a guide. From there it is basically drawing what I have sketched but now using much more detail with precise line weights, color and texture.
Sometimes on selected projects, I'll just put the product in front of me and draw it directly in Illustrator. Other times, I’ll export to Photoshop to add effects or additional texturing and blending.
There are a few medical illustrations that are created in Photoshop from start to finish.