Before you start

Residence card with the address printed on it
A Japanese bank account or credit card, for most postpaid plans
An unlocked or eSIM-capable handset supporting Japanese bands
A Nagoya address, for fibre

Step-by-step

1
Bridge the first fortnight with a travel eSIM
A data-only eSIM bought before you fly covers you until the residence card and bank account exist, which takes a couple of weeks. It removes the pressure to sign something unsuitable in week one.
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2
Go to an MVNO or a carrier sub-brand rather than a flagship plan
IIJmio, mineo and the carriers' own low-cost brands run on the majors' networks at a fraction of the price with no long commitment. Flagship plans only make sense if you specifically want the bundled services.
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3
Expect a Japanese payment method to be mandatory
Most postpaid plans need a Japanese bank account for direct debit or a Japanese-issued credit card, and foreign cards are frequently rejected. This is why the bank account comes before the phone in every Japanese relocation sequence.
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4
Ask what physically arrives at your door before you sign the lease
A building advertised as fibre-connected may run fibre to the basement and VDSL over copper to each flat. Aichi's large stock of public corporation and UR estate housing is particularly prone to this. Ask specifically whether it is fibre to the room, and treat it as a lease-signing question if your income depends on the connection.
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5
Get the ETC card sorted if you are going to drive
The transponder card that discounts expressway tolls is issued against a Japanese credit card, which requires the bank account. In a region where the expressway network is part of ordinary life, this is worth starting early rather than at the toll booth.
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6
Keep your number when you switch
Japan runs mobile number portability and has simplified it so that you can move between many operators without obtaining a transfer code first. Never cancel the old contract before the switch has completed.
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Documents you’ll need

Residence card with address
Japanese bank account or credit card
Passport, occasionally requested
Proof of address, for fibre installation

Things most newcomers don’t know

The bank account gates the phone, not the other way round.

Most Japanese postpaid plans require a Japanese bank account or a Japanese-issued credit card, and foreign cards are commonly refused outright. Because the bank account requires ward registration and ward registration requires an address, the sequence runs address, ward office, bank, phone. Attempting it in any other order produces most of the frustration people report from their first fortnight in Japan.

Source: community-reported

MVNOs run on the same towers for a fraction of the price.

Government pressure on the three major carriers produced both low-cost sub-brands and a healthy MVNO market, all using the majors' own networks. Coverage and speed are effectively identical outside peak congestion. Anyone on a flagship plan without needing the bundled extras is paying several times more than necessary for the same service.

Source: Ministry of Internal Affairs and Communications

Aichi's cheap estate housing is cheap partly because the line into it is copper.

The prefecture built an enormous amount of public corporation and UR estate housing for the manufacturing workforce from the 1960s onwards. It is spacious, well-priced, requires no key money and often no guarantor — and a great deal of it is wired with building-wide VDSL that caps your connection far below the plan you signed. Ask about fibre to the room specifically; the answer is frequently no and it is not negotiable.

Source: community-reported

Nagoya is on the 60 Hz western grid, which matters for second-hand appliances.

Japan is split between a 50 Hz east covering Tokyo, Tōhoku and Hokkaido and a 60 Hz west covering Nagoya, Osaka and Kobe, with the boundary running through Shizuoka. Mains is 100 V throughout, the lowest in the world, so European appliances need a transformer regardless. Anything with a motor or a timer bought second-hand from someone moving down from Tokyo may not behave correctly here.

Source: community-reported

Common mistakes to avoid

Trying to arrange a phone contract before ward registration and a bank account exist.
Paying a flagship carrier price when an MVNO on the same network costs a fraction.
Signing for cheap estate housing without confirming whether fibre reaches the flat itself.
Buying a 50 Hz appliance second-hand from someone relocating from Tokyo.
Cancelling an old contract before the number transfer has completed.

Some of this may be out of date. Spotted something inaccurate? Help us keep it right for the next newcomer.