Step 1: Set up: choose your image, buy materials
1. A laptop
2. A high resolution image (resolution should be at least as high as your laptop screen)
3. A white gloss self adhesive A4 vinyl label suitable for inkjet printers (I ordered mine from eBay http://www.ebay.co.uk. Ten sheets cost me 10GBP).
4. Some sticky back plastic. I used the stuff libraries use to cover their books, bought from my local stationers Rymans http://www.ryman.co.uk/. A huge roll cost only 2GBP. It is thin, durable and tough.
5. A colour inkjet printer
6. A scanner is useful but not necessary
Step 2: Find the correct image size
If you have a scanner, scan your laptop. Using an image manipulation package that supports layering (e.g. gimp http://www.gimp.org/) load the scanned image. Add a new layer and in this layer draw a box over the scanned image that marks the intended edges of your laptop skin. Make sure you curve the corners of the box to match the contours of your laptop if you wish to do so. Delete the layer containing the scanned image, so you are left only with the outline of your laptop skin. Now load a new layer with the image you wish to use as a skin. Resize and position the image so that the outline of the skin contains the image correctly. Now merge the layers. You should now have an image with the intended outline of your skin superimposed which will help you when you need to cut the image out.
If you do not have a scanner, you will instead need to resize your image so that when it is printed it will be the correct size for your laptop. Use an appropriate image manipulation package.