Check these out! These are what McQueen's printed fabric and pattern pieces look like from the Spring/Summer 2010 collection!
In regards to your question, I know that prints can be made fairly easily on Adobe Illustrator and Adobe Photoshop. Is that what McQueen used for his prints? I don't know. But I know it can be done, as many of my friends are taking a class here at Parsons in which one project requires them to create a pattern on Illustrator and print it on fabric.
The easiest way I can think of, is to start with a simple square, any size, 4"x4", etc., on Photoshop. Then, do whatever you want within that square. You can work with a photographic image and manipulate it, you can draw an image, you can paint abstract shapes and strokes, anything. Once you've created your base square, you can import it into Illustrator and duplicate that square as many times as you want. You can rotate the square, you can place them in a sequence (if you use multiple base squares), you can adjust the sizing of your squares too. It's really just a matter of playing with your square.
Try looking at this blog post for some more detailed guidance:
And for something a little more advanced:
Back to McQueen, though. His prints are far more technically advanced that what I've described, because it takes into account how the pattern will look once it's used in the final garment (additionally, I believe his fabrics were not printed with the pattern, but instead, the pattern was actually woven, which is mind blowing, really). Now, if I were to go about doing this in a more elementary way, this is what I would do. Create a muslin mock up of whatever it is that you want to make (start with something simple, like a beginner's top or skirt). Then draw with sharpie a basic idea of how you want your pattern to curve and wrap around the shape of the body when the garment is worn.
Then, take apart your garment and you will see how the pattern will look, generally, when it is laying flat. You will notice breaks in patterns where darts belong, etc. Then, ideally, I would scan the pattern pieces, but because you won't always have access to a large scanner, photographing each pattern piece individually from above will be just as good. Just make sure when you photograph it, it's not at any angle...you need to do it from straight above. Then on Illustrator, import your scan or photo of your pattern pieces and using the Pen Tool, create an outline of your pattern piece and the guidelines for your print. From there you can work on Illustrator or Photoshop creating your print that follows the guidelines you made. Then when the pattern is finished, it can be printed on fabric, and you simply cut out the pattern piece, sew the darts, etc., and voila!
I've never done that before, but theoretically, it's how I would. Whether or not that is the easiest way, I'm not sure, but again, it's how I would go about doing it now.
That may have been a mouthful, and if you haven't had any experience on Photoshop or Illustrator, then likely none of what I said made any sense, but if you have any more questions, I'm more than happy to help!