If you arrived _this_ close to the start there are plenty of things you can
do to solve people's problems for them, simply by giving information.  You can
tell people that Ryouga is P-chan _before_ Ryouga arrives, for instance, be-
cause Akane hasn't taken P-chan as her pet yet.  You can warn Ranma not to de-
feat Shampoo in combat as a male, you can say "I think you'd better check up
on the seal on Happosai's cave", you can tell Ranma how to defeat the shishi
hokoudan, etc.

Of course, one catch is that the more you change things, the less events are
going to go as you know them.  If Shampoo doesn't fall for Ranma, for in-
stance, Cologne doesn't come to Japan or try to teach Ranma, and Ranma never
learns the tenshin amaguriken; and a lot of other stories simply don't happen.
(Heck, you may have to teach Ranma the tenshin amaguriken or other techniques
without practicing them yourself, just from memory of the episodes.) On the
other hand, some other events will probably happen as scheduled even if you
change lots of stuff before them, such as the arrival of characters like Ukyou
or Nodoka.

The fact that you're an author stuck in a fanfic is almost irrelevant; the
story is pretty similar to the more typical sci-fi version (such as Lest Dark-
ness Fall) where the hero gets all this knowledge of the future from being a
time traveller instead of an author.

Personally, I'd also take a little time to get taught some martial arts
techniques by the regulars.