“Writing forsoothly” is the term we like to use around the house for all the different varieties of bad pseudo-archaic diction that infest modern fantasy — historical and created-world fantasy in particular. J. R. R. Tolkien is undoubtedly to blame for a lot of it, because his characters do like their elevated language; what unobservant readers miss is the way that Tolkien modulates his characters’ dialogue, moving effortlessly from plain vernacular to almost-archaic high formal speech and back again, depending upon the situation and the company. Strider the Ranger has a much commoner way of talking than Aragorn the Heir to the Throne of Gondor, but at the same time they’re both recognizably the same guy.
It’s probably unwise to play with writing in extreme forsoothly unless you can at least approach Tolkien’s level of skill and language-awareness. It’s a lot harder to do than it looks, and the failure mode is dire. But if you’re determined to give it a try — and nobody ever makes any progress in this game unless they regularly try things that they aren’t certain will work — there are a few things it will help to do first.
One: Ask yourself, “Is this really the direction my writing talent lies in?” and answer it honestly. If your interior Magic 8-Ball refuses to yield up anything more specific than “reply hazy; ask again later,” find a kind but honest friend and ask them. Kind, because you don’t want your self-image pulled down and stomped upon with hobnailed boots; and honest, because you’re not asking them for sympathy, you’re asking them for the truth.
Two: Prepare yourself. Read genuine period or formal writing until it dribbles out of your ears. If you start talking in Shakespearean or Regency English at the breakfast table, you’re probably ready. And a good thing, too, because at that point your friends and family are either bored stiff with your project, or convinced that you’re going nuts.
Three: Stop researching and write. Don’t worry about getting all the nuances down perfectly; you can always polish the heck out of the language in your second — or third or fourth or fifth — draft.
Four: Go find that kind but honest friend again. This time, ask them if the archaic or formal language in fact worked; and ask them, also, whether they think you got it right but took it too far. As with so many other things in writing, a light hand is best.
(For an interesting example of archaic diction done well in an unexpected venue, check out the historical romance For My Lady’s Heart, by Laura Kinsale, now available again in e-book format after a long while out of print.)