First thing you do is head on over to
userscripts.org and do a search
on rotten tomatos to see if anyone has already made your script for
you...
A quick search on 'rotten tomatos' gets you a few IMDB and rotten
tomato related scripts. This one [
http://userscripts.org/scripts/show/12897
] seems like the best, it adds information from the Rotten Tomatos
site into the IMDB page. Doing multiple searches on rotten tomatos,
wiki, and imdb should get you a few more scripts that might be closer
to what you want.
If you are still intent on rolling your own, you can start off by
installing the script that is closest to what you want, I saw a few
that add links to the rotten tomatos site to other sites, like pirate
bay. You can install one of those (a simple one is better) and can
start poking around and seeing what parts of the script correspond to
adding in links. From there out google is your friend in terms of
looking up info on specific Javascript functions you encounter, and
for things like XPath and regular expressions, both of which you would
probably need for this script.
Once you have the general idea down, you can always come back here to
ask more specific questions.
Have fun!