My colleague uses only his right hand to type Japanese into his computer. With his hemiplegia, a qwerty keyboard has a few too many keys.
I believe that the keypad (ten-keys) on the right side of a keyboard can be a good start for him to learn touch typing. Here's how to configure a Windows PC for keypad Japanese entry.
Japanese language use a few thousand kanji, chinese ideographs, and about forty kana, japanese phonetics. All kana and their modifiers have been assigned an alphabetical representation called romaji or romanized kana. When you enter these alphabetical romaji, software such as Anthy, Microsoft IME, or ATOK translates them into kana and kanji with some trivial but error-prone user interactions.
Most romaji assignments are combinations of one consonant followed by one vowel. Example: sashimi becomes sa-si-mi.
Vowels can be used without consonants to represent kana.
I have assigned two keypad strokes for each romaji, consonant first followed by vowel.
First stroke keys 7 8 9 4 5 ... for consonants <none> k s t n h...
Second stroke keys 7 8 9 4 5 for vowels a i u e o.
"Huh? Why not 1 2 3 4 5 but 7 8 9 4 5," you would ask? When you visualize telephone buttons or calculator buttons, you get 1 2 3 4 5... assignments for "a i u e o." Some people still remember when they sent romaji messages to a pager using telephone keypad. Some modern smartphones may also accept good old pager key assignments. I borrowed this smartphone arrangement of 1-2-3... onto PC keypad, that's 7-8-9.
Sashimi was sa-si-mi in romaji, their keystrokes are 97 98 18(without intervening blanks). See the entire assignment table on the Japanese side.
Only the basic rules are covered in this document. Missing from this document are: modifiers, packed key assignments (not enough number keys for all consonants), romaji to kanji translations, among others. Send questions to me at claude dot k at spamcop dot net.