Step 2The first incision
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The first thing to do is remove the horrible plastic headband that came with the headphones. I briefly considered doing this in a surgical manner, and then just broke them off. If you want to be technical about it you could dremel/hacksaw/melt them off, but at this stage you probably want to keep at least part of the headband attached to give room to fine-tune later.
Alternatively, if you want to just use the speaker (the inner circular bit here) and mount it yourself, now would be a good time to cut it out.