While the design of headphones vary, in general the principles of taking stuff apart is the same - hope for screws, glue is annoying, and its only really yours if you can take it apart and put it back together. In this instructable i'll be taking apart an old (and trusty) Sennheiser EH150. Many newer models use the same basic design, such as the HD202.
While, your aim might be to repair the headphone, there are also many mods you can do (such as filling the space with a suitable material), or you could just dismantle it for parts (in which case you can be a lot less careful.
First, you may want to detatch the headphones from the headband if possible. In this case, i could just pop them out.
And yes. I'm entirely aware of how terrible the cushion looks. I'll be replacing or repairing them once this headphones are eventually recabled
Step 1: Strip It Down!
In this case, the cushion is attached to a small pad that snaps into the base. If it uses a sponge, its even easier. Note the 4 screws - this makes things easy, since its less likely something will go wrong- as it would if you had to very carefully pry apart tabs, or if it was screwed together. In any case if you want to actually fix it, patience, care, and the lack of *crack* sounds go a long way,
In this case undo the screws, and crack open the wonderous gooey insides...
Step 2: Headphone Innards
At this point, the world is your oyster- you can recable it, fill the inside with polyfill or blue tac (which allegedly improves some headphones), fit in a subliminal messaging device... the possibilities are endless. Once you're done, close it back, and its good as new.