HOW THE HECK DO THESE JOURNALS WORK?
↪A primer on the Hogwarts journal system
The journals that your characters use for communication are more or less like a cross between Tom Riddle's diary (sans evil) and a fax machine. Whatever is written in one journal will show up in other journals more or less exactly the way it's written. Yes, this means everyone has to read your character's chicken scratch. It also means that strikeouts are literally written, and then struck out. Whether it's just a single line or an inky scribble forest is up to your character.
It also means emoticons should be few and far between, unless your character is that much of a dork to write them out by hand (and is familiar enough with computers to know what they are in the first place). Of course, regular-type doodles are perfectly fine, if your character is the type.
✒ How does posting work?
Your character's journal has two sections: one for their own entries, and one for reading the entries of others (analgalous, obviously, to their own livejournal page and their friends list page). They can flip through the front section to read their own past entries, or flip through the back to see what other characters are saying. Entries are posted one per page (or one per several pages, if it's a particularly long one, but two short posts will still have their own pages), and the pages are labeled with the name of the student whose journal it is. Making entries private or filtering them is done via a (fairly simple spell; touching the wand to the page, twisting it like locking a door with a key and saying "claustro") that determines which journals the entry will or won't show up in.
✒ How do comments work?
Comments are written on the same page as the journal entry, below the body of the text. They can then continue onto the back of the same page, or spread to fill multiple pages if necessary (blank pages will move in between a given entry and the next one to facilitate this. Hey, it's magic.) Comments are made private in the same way as posts, although once a comment thread is under the privacy spell, all further comments in that thread will be set to the same privacy level automatically, unless the spell is deactivated or a more secure spell is cast.
✒ How does hacking work?
If an entry or comment is "hackable," that means you screwed up on the spell. Hackable does not mean readable. Other characters will be able to see that there is text that they can't see, but they'll have to do some spellwork of their own to actually read it. Of course, the more hackable an entry is, the more possible it is to make out bits and pieces without additional effort. Privacy can also be put on just parts of a comment, or on top of something struck out for extra protection.
✒ How do voice posts work?
Voice posts are similar to Howlers, in that they record your voice into the parchment and repeat it later. A voice post will autoplay when a character opens their journal to the page it's on, unless they silence it with a spell. Voice posts can be repeated as many times as you'd like, simply by closing the page and opening it again.
✒ How do video posts work?
They don't. However, a character can spellotape pictures into their journal (both gif images wizarding photos and jpgs and pngs muggle ones) which will then show up in the journals of the other characters (although, like a fax machine, if you stick a picture in with tape, other characters can't unstick the tape and take your picture. It's just an image).
✒ How do icons work?
For that, I'm not entirely sure. Icons are not literal images of what a character is doing (if Alfred is eating a donut in his icon, he isn't necessarily eating a donut in real life-- although it is very likely), but they represent their emotions in a way that the other characters can pick up on. Because tone is generally hard to determine through text, icons are a way of conveying that tone.
✒ I have a question and you didn't answer it!
By all means, ask away. I tried my best to be comprehensive, but I know there's things I've overlooked, so feel free to leave a comment and someone will answer it.