There seems to me to be two methods that one create a Howler. Due to the fact that all Howler's (in the books at least) are red envelopes or letters, whilst normal letters and such are made of white parchment, it seems logical to assume that the a Howler is made from a special type of paper that's been enchanted to convey the senders message as loudly as possible, I'm going to call this Howler Paper from now on.
The first way I thought of was that the Howler paper is written on, just like a normal letter, and then after a spell of some kind, it is 'activated' and then quickly attached to an owl who flies it to it's recipient. One of the main problems I see with this way is that the Howler Paper wouldn't really know what to sound like, the only time that a voice could actually be focused on it is when the activation spell is said, and then would it even be able to hear it at that point?
This bought me on to my second idea and the one I think more likely. The Howler Paper is less of a letter in the way we think of them and more of a middle-man between the sender and the recipient. Instead of writing the message the sender would cast an activation spell of some sort and then say the message to the Howler Paper, perhaps there also being spells of some sort that make it understand what tone the sender wants bits to be in, before casting another spell that let's the Howler Paper know the message is done before then, once again attaching it to an owl who goes to find the recipient. Although the requirement of two spells does make this method a bit more complicated.

Of course if neither of these ways are correct, it could just be a spell that transforms a normal letter into a Howler, but once again how does it know what to sound like?

Assuming there is such a thing as Howler Paper in the Harry Potter continua then I would guess that that would be the sole thing specifically needed to make a Howler, especially if the second method is true. Where one would get it? I would assume shops like Scribbulus Writing Implements in Diagon Alley, or Scrivenshaft's Quill Shop in Hogsmeade. For Agents, I assume the Room of Requirement would be able to provide it.

Whether or not the Agents know how to do it or not could prove funny as they try a load of different ways throughout the fic. I think that in the end it may not necessarily matter if we know how they ended up doing it, seeing as the end result is known to be canonical so long as it seems that what they did would seem to be canon and would work with what resources canon gives us.

I hope this makes sense, sorry for any and all grammatical mistakes I've made along the way

Storme Hawk

Unless you're suggesting that they'd have to be the entire series, which we know not to be the case because nobody carries around the entire boxed set/full book series of Harry Potter. Is there some sort of critical mass of canon or something? If so, how do comic books work?

Besides, I quite like the image of a brace of agents sat in a ring of candles diligently copying out key quotes from their canon of choice onto thin strips of paper. It could be a really good way to show character development without any dialogue, a major moment of bonding between the two agents conducted in respectful, meditative silence... yeah, that'd be cool.

It's almost a shame I can't write it with Wobbles and the Notary. =]