Make a sewing pattern by covering an object with a plastic bag and wrapping in packing tape.
I made a sewing pattern for a bike helmet cover, but this technique can be used to make a pattern for just about any object, even people!
Step 1: Cover and wrap the object
Cover the object with a thin plastic bag. A dry-cleaning bag is a good weight for this. Now wrap the object in packing tape. I cut lengths of around a foot (30cm) to make it easier, but the important thing is that you want to wrap the object tightly! Your packing tape will get lots of wrinkles - that's fine. Baggy bits? Just go over them again with more tape. Make sure the entire object is completely covered.
Step 2: Make some guide marks
If you know where you will definitely want a seam, or if you want to mark in special features of the object, do it now. If your object is symmetrical, mark a centre line now. If you want to be really accurate, measure the object to find the centre line.
I marked the ventilation holes on the helmet and the stripe I will eventually put down the centre. I also marked along the bottom edge of the helmet.
Step 3: Cut it free
I was able to cut the helmet free by cutting along the mark I made at the edge of the helmet. If your object is not as conveniently-shaped as my helmet, you might have to carefully cut down the middle or cut in a few places. If you have to do this, draw the lines first where you're going to cut. Place marks along this line as shown in the next step (step 4) so that you know where your cut lines match up.