To apply for a visa renewal ('extension of period of stay') in Japan, obviously there's an application form. There's a damn form for everything here.
Whether you fill it out in English or Japanese is up to you. But if you've been learning the language for a while, like me, people will expect you to use Japanese.
Considering it's only a 2-page form, getting everything right can be tricky.
Know your place (at work)
In the space where I'd written my proper job title, the admin person at work suggested I should erase it and just put 'employee'. There were no problems with doing that - I got my new visa. No delays, no questions.
Immigration doesn't tend to care what exactly your job is. They care that you're here legally, and that you're doing work under the scope of your visa category. Actual title... meh.
Not putting down your full job title isn't going to count against you. But for a work visa, definitely write that you're employed.
Know your place (in Japan's visa hierarchy)
On the other hand, for 'current visa type' you need to write out the whole thing.
Let's say you have an 'Engineer / Specialist in Humanities / International Services' visa. Now, it's unlikely you'd be doing all 3 of those. Pretend you're an engineer for a second.
Technically, your status is under the 'Engineer' bit. But that's not the full name of the working visa you hold. And the full name's what needs to go on the form. So don't write 'Engineer' and hope to get away with it. Be accurate down to the letter.
Try and remember your university days
When you applied for your first Japan work visa, immigration wanted your degree certificate, remember? You handed it over, they went 'oh yes, this person did graduate', and that was that.
On the renewal form, you have to put down your university, general subject, and graduation date once more. Let's hope you still know what those were.
Why is that info still needed? I'm not sure. I suspect it is, in fact, a memory test.
Have a really good reason to renew
There's a space on the form to write your 'reason for extension'. This answer needs more consideration than you'd think.
'So that I can keep on doing my job' doesn't count. (I asked.) It's a reason, and a valid one for you, but not inspiring enough to convince immigration.
You've got to get a bit creative. Talk about how you want to carry on... learning. Growing as a person. Sharing culture and experiences that are uniquely Japanese.
Okay, maybe not that flowery - but not far off.
Keep copies of EVERYTHING for next time
Go ahead and make a copy of each form once you're done. You have to submit originals, so it's good to keep copies for your own reference.
When I say 'your own reference', I mean so you can write the exact same things again for the next renewal. You're more likely to be given a 1-year visa anyway. Better safe than sorry.