Step 1: Materials and tools
Old EI transformer which you don't mind destroying it
Hammer (the must have tool in every workshop :D)
20 minutes of your precious time (5 minutes for reading this Instructable, and 15 to build the demagnetizer)
Step 2: Taking apart the transformer
If you are more experienced transformer destroyer you will wont be needing more than 10 minutes to do this
Step 3: Geting back the transformer
You will wont be needing the I laminations for this, which means you will need to throw them, or do something else with them. You will put back all E laminations in place all turned on way (not like when you disasembled it) and put just the primary (if you can't separate the primary out of the secondary you can leave it like that, or unwind the secondary) and...JOB DONE!!!! now you are ready to try it out.