Knic, knaec, knoc --- it's me!
Ille illa illud basically follows the 1st declension for the feminine and the 2nd declension for the masculine and neuter, so if you can decline an adjective like magn-us -a -um (or nouns like anima, animus, and templum) most of its forms should already be familiar to you.
The genitive and dative singular, in -īus and -ī, may seem odd, but they are in fact regular endings for pronouns: quī, is, iste, and ipse all decline similarly, as indeed does hic, and as do certain adjectives like ūnus and tōtus.
The nominatives ille and illud are the only other apparent oddities, but again these seem less odd once you've seen other pronouns. Ipse and iste have the same masculine nominative ending, and quī (quod), is (id), and iste (istud) all have neuters ending in -d.
Hic haec hoc roughly follows this same pronominal declension on the stem h-. The starkest complication in its declension is the addition of the ending -c to many of its forms (apparently the vestige of an old suffix, -ce, which gave extra demonstrative force to a word). The expected forms hum, ham, hui, hō, and hā are still easily visible behind the actual forms hunc, hanc, huic, hōc, and hāc.
The appearance of a -u- in the genitive and dative singular forms huius and huic wants some explanation, though you won't get it from me. Some solace can be found in the fact that they look like the queer forms of cuius and cui in quī's declension.
The nominative forms hic, haec, and hoc are properly strange, but will be remembered easily enough as being the "name" of the pronoun.
This leaves the neuter plural haec as the only remaining departure from regularity, but this is another one that is resolved upon meeting the other pronouns. It seems to be a feature of the pronominal declension that any pronoun with a feminine singular ending in -ae has a neuter plural ending in the same.