Yes, you go to the Post Office with cash.  They 
will not accept credit cards, or even traveller's 
checks, to buy an IPMO (I found this out the hard 
way, when I was traveling around the USA with a 
laptop and continuing to shop for cels as I 
travelled, and the Post Office in the town I was 
in one day told me that "American Express 
Traveler's Checks are NOT as good as cash" (in 
spite of what the ads tell you)).  There is a 
charge for the IPMO, which if memory serves me, 
is something near $4.00.

Ask for a "pink IPMO", and be sure to tell them 
it is for Japan.  IPMOs to some less-connected 
countries have lengthy clearing procedures that I 
believe happen in St. Louis, but IPMOs to Japan 
are easy and efficient.

Buy the IPMO, fill out the left side completely 
with the sellers info and the right side 
completely with your info.  Stuff it in the 
envelope and you are good to go.

Most importantly, keep the sales receipt from the 
Post Office and the stub from the IPMO, and do 
not lose them until you have your cel.  My 
overseas mail has been lost on occasion (maybe 1 
in 25 times), and with the paperwork in hand, the 
Post Office refunded my money for the lost IPMO.  
Without the paperwork saved, I would have been 
out of luck....