It's a straightforward process, somewhat.
Notepad assumes text is written using ASCII, which means characters with hex values 00-1F get interpreted as garbage or stuff like line breaks / tabulations / etc.. if not outright omitted. Space is 20, 1=31, A=41 and a=61.
By necessity, Japanese games (either translated or not) usually don't follow that pattern, so these hex values mean something else entirely.
For example, in one game, 00 might be a space. In another, it might be the first hiragana Japanese character "a". In yet another one, it could be the digit zero. In one, it might be not even a real character but a control code to tell it to stop reading text, or do a line break... And so on...
So you need a file with what hex value gives what character.
That file is called the table.
Assuming we made a table for a game using ASCII (which would be stupid, since it's supported by default on any hex editor that didn't load a table yet), that table would look like this when opened in notepad (because it's a txt file disguised with a tbl file extension):
With hex editors like WindHex, you can open the ROM and then open its table file, and in some part of the rom you'll make out the text clearly because you have the table loaded and telling you the right values for text characters.
There are tools to make such table files.
You search for a word you know it to exist in the ROM. It usually works only with A-Z / a-z characters.
It expects for example if you search for ALPHA that the first and fifth character (A) have the same value, and that the second (L) and third (P) have a difference of 4 between their hex values (because alphabetical order).
Latin alphabetical order is expected.
Try monkeymoore a bit with games with some English text (check the font is complete and in alphabetic order first! if possible) to get familiar with it.
Japanese kana order is unexpected.
An order exists indeed.
However there are "variants" of some letters, and those don't have a fixed order so they either are all lumped in the end, or only some, or mixed with their original letters.
monkeymoore is dumb. So in cases you want Japanese text, you can tell him the order used in that particular game.
I recommend you hack stuff already in a latin language first.