"how I used it" (noun phrase) comes right after "today" (noun).
The phrase "how I used it," which need not be analyzed as a noun phrase, does indeed come right after "today" (which is here a reference to the word "today" rather than an actual use of the word); however, it really relates to the noun "usage," or to the noun phrase "usage of 'today.'"
I'd like to know the reason why a preposition isn't needed between "today" and "how I used it"
Excellent question. I'm going to go out on a limb. Although the phrase "how I used it" can function as a noun phrase (e.g., in "He likes how I used it"), it can also function as an adverbial clause, and that's how I'd analyze it in its modification of the noun phrase "usage of 'today,'" which is a derived nominal. Compare:
(1) He used the word how I used it.
(2) He used the word where I used it.
(3) He used the word when I used it.
(4) He used the word why I used it.
Those sentences mean different things. My point in giving those examples is to compare the syntax of the
how-clause in (1) to the syntax of the
where-clause in (2) and the
when-clause in (3). In each case, the function of the clause is adverbial, as can be seen by the fact that the sentences answer these questions:
(1') How did he use the word?
(2') Where did he use the word?
(3') When did he use the word?
(4') Why did he use the word?
Notice that you could not grammatically add a preposition before the
how, where,
when, or
why in any of those examples. Also, in the first three, the
wh-word plays a role in both the main clause and the subordinate clause simultaneously. Semantically:
He used it how1; I used it how2; how1 = how2.