Japanese people can all read roman letters, butthe romanization systems for showing Japanese words are a bit cumbersome and so students learn hiragana, the syllabary, as soon as they get started studying. So I have been able to write in hiragana for years of course, without learning to type. After a year's worth of false starts, I learned to type in Japanese in an afternoon.
Here is all I actually had to do:
1.Activate the Windows Japanese Input mode
This step is really easy to find out how to do. Control Panel ->Region and Language -> Keyboards and Languages -> Change Keyboards -> Add -> Japanese -> Apply, and then in the task bar switch between EN and JP at will.
2. Label the keyboard
I just pressed every key and wrote down what it rendered.
3. type the gojuon/syllabary in order
Realize the keyboard is not actually a confusing mess of characters.
So here is the gojuon, which is the alphabet analogue. It's arranged by vowels across and consonants down.
Here is how it applies to the qwerty keyboard. It is mostly clusters with a bunch of outliers, I do not know why. Also, just a note if you are reading this and fairly detail oriented but haven't studied japanese: chi and tsu are in the t category, shi is in the s category, and fu is in the h category. I don't know the reason for most of those, but fu is because the japanese h is like an h/f hybrid.
My two workarounds before I learned to type yesterday were to use my phone, or type in English into google translate and then copy/paste. The phone keypad is actually perfect for the gojuon. It works like the T9 system in English. Multiple taps take you across the columns of the gojuon, and there is one key per row.