Okurigana literally means "sending off" kana. An important thing to note while
learning characters is that when combining kanji and kana to form words, is that the
okurigana MUST start where it is possible to inflect the word.
Thus the examples above are slightly incorrect: iku can be inflected as ikimasu, and ikemasu (can go), and therefore the okurigana must start after the initial i. Ikimasu is written I(kimasu), with the characters in the brackets in kana.
Another rule when forming okurigana with verbs is that the okurigana must allow the chinese character to be distinguished from other yomi. Taberu is a case in point. The full writing for tabemasu makes the okurigana begin at the "be", thus ta(bemasu). Why is this? Regular inflections of taberu only change the word after the "be", e.g., taberarenai, tabete, so why isn't tabemasu written tabe(masu)?
The reason is that another word exists that uses that character, kuu, which incidentally, also means "to eat". To distinguish which word is being used, the okurigana include the second syllable, "be" in the case of ta(bemasu), and "i" in the case of ku(imasu).
This rule is particularly important with such words as (the kanji for) iku, which is one of the characters with the most yomi in the Japanese language.
The okurigana for adjectives is much easier. There are only 3 types of okurigana that are used: the -na, ending for na adjectives, and -shii and -i for -i adjectives.