You take it to the main post office in your area. All the packages are taken back to the big post office in your area - for example, if you live in a small town, it should have been sent back to the main town post office. A larger city will have multiple "main" post offices - you`d just go to the main one for your area.
They`ll ask for identification, but that`s about it.
On the other hand, you can also fill it out and drop it into a post box - but I doubt that would work as if you can`t read it, it would be very hard to write it!
If you PM me with most of your address, I`ll look up the main post office in your area for you.
Edit: I just noticed you`re in Tokyo - you`d go to the ku post office if you live inside the main city limits. If you don`t, then it will most likely be your chou post office.