Excuse me in advance for being a big nerd who doesn't use this site much, haha.

So, what's the deal with custom items? I've heard tell of them, and from what I understand they're equippable things for your avatar that you somehow make with diamonds, but know very little else. I've looked in the forum but I can only find threads with users selling their own, no guides regarding them. Could someone give me a rundown?

Usually Krinadon's post announcing the monthly avatar items gives a pretty good run down. The post for this month is here (there is a section titled "Ordering Custom Monthly Items"). Let me know if you have any other questions.