Scripts are nightmarish but you should just be able to copy+paste out of that forum post and get it working fine. Your template also should have some scripts already so you can just tack it on to the end of that probably.
Also quick tip: if you dont want to use the uppercase macro every time you put a pronoun at the start of a sentence, you can actually work it so that the backend still has the pronouns. You'd just have to add a few lines that are like <<set $her_uppercase to ucfirst($her_lowercase)>> after each pronoun set.
Honestly though its kinda a crime that Twine doesn't just have a way to do this normally. Like, in a text-based game software you'd expect to have basic text manipulation functions baked in lol. Oh well :P