Found this tutorial on Instructables by user Megg. This tutorial shows you how to use a plastic bag, packing tape, some paper and a marker to make a pattern for a custom bike helmet cover. The instructions take you as far as making a prototype cover out of muslin or scrap fabric so you can make sure it fits the helmet. When it to fits to your liking, you simply use your pattern to make the final cover out of whatever fabric you choose. Shown in the picture above is a cover made out of grey fleece.
If you’re handy with packing or duct tape and have a bit of experience sewing, you should be alright.