When I last replaced my windscreen wiper blades, I put the old ones in the bin, but I knew they wouldn't stay there long. Sure enough I had this simple idea. When I replaced them on my new car, I decided to do the same and take some photos while I was at it.
Pair of old windscreen wiper blades
Steel wire (not stranded)
Step 1: Take the rubbers out
Just pull them out, the metal strips too. Simple as.
The metal strips could possibly be used instead of the steel wire later.
Step 2: Cut slots and slot blades together
Cut two slots in the middle of the top of one wiper blade, the same distance apart as the two sides of the blade. I used a Dremel with a cutting wheel 'cause I didn't have any hacksaw blades, but a hacksaw should do the trick. Don't cut all the way through. If there are holes in the blades already, it's fine to cut into them, or not.
Slot the other blade in to the slots at right angles. Depending on the accuracy of your cuts, the tension may or not hold them together. If so, great, if not, never mind, the wire hook will hold them together later.
Step 3: Add paper clips
Clamp a paper clip in each of the little hooks that held the rubbers by forcing them together with pliers. Make sure the paper clips are the right way round - the photo needs to be able to slide into it.