The core rulebook describes Parma Magica as a "special ritual (not a ritual spell) that takes about two minutes to perform." To the best of my knowledge, this is as detailed at the books get in describing the ritual.
Now, seeing as how the description specifically says that it is not a ritual spell (to avoid the Vis cost that all ritual spells come with, I would assume, if nothing else), is it possible to perform the ritual under circumstances that don't exactly lend themselves to successful spell-casting?
The example that (naturally) comes to mind for me is a Bjornaer who is in his Heartbeast form when the time comes to renew his Parma. Is he capable of performing the (or a) ritual when he has no human voice, and only hooves, flippers, wings, paws, what have you? Or is he required to take his human form to perform it?
I can actually see this as a situation where the Bjornaer would have come up with a ritual that can be performed in either form, or even two separate rituals (one for each form) to keep himself from being exposed, but I'm not 100% sold on it.
I can also envision situations where a maga from another House may, for whatever reason, be unable to cast spells normally but still needs to perform her Parma ritual (she has a broken arm in a sling, or she's held prisoner and bound and gagged, or what have you). Would she be able to maintain her Parma, or is she s.o.l.?
Oddly I have always considered Parma to be done like the Bene Gesserit Litany Against Fear. A chanting mantra to focus the mind, which needs you only to be conscious to perform. I may have missed a pertinent reading though.
As I can tell, all a magus needs is two minutes to focus ones Gift to mentally weave the bonds of protection. I think of it as the strands of your magical knowledge whip about you chaotically and energetically but when you do the ritual of Parma, you are in essence weaving the strands of magic about you as a protective shell, as you increase your puissance in Parma, it is as if you become more nimble as you mentally constructing connections that strengthen and reenforce each other making the shell of protection a even tighter weave, stronger, denser.
But this is only how I describe it... the truth is quite ambiguous as Peregrine says.
My inclination is to read it as requiring fairly elaborate rituals, including words and arcane gestures, much like Ritual spells. I don't think props are needed (I do like to require them for ritual spells), but still - that does seem to be the intention from my reading.
I grant that this creates a problem for Bjornaer. I suggest a House Mystery adapting the Parma to be cast in an "internal" ritual, as interpreted above.
I also grant all of this is only my personal interpretation, and the rules are ambiguous on the issue.
While I can't address the OP's specific question, I have been thinking about Parma a lot for my new saga, and I think that --whatever the ritual is -- it cannot be solely made up of physical gestures and incantations. An apprentice has the daily opportunity to observe his master raising his Parma, and even if most magi were to not permit their Apprentices to observe them, some would certainly do so anyway, and if they could learn Parma though observation -- memorizing the words and copying the gesture -- then surely this would have happened at some point. But the rules do not permit this to happen. No apprentice who follows the character creation rules can learn Parma until they pass their Gauntlet and a magi teaches them "the secret" to Parma, whatever that secret is.
I have decided that, in my Sagas, Parma is a combination of physical ritual and mental spell. The physical component creates a metaphorical "lock" around the magus, one unique to him, while the mental part of the spell is the "key" to that lock. The final part of the ritual involves the magus using his mental key to close the magical lock.
I do this because I want to preserve a physical component to Parma. I want it to look cool. But it must have a mental element which is relatively easy to instruct but unique to every magus. I have a Bjornaer magus in my game, and I would call the performance of Parma like any other act of magic. All hermetic magic requires gesture and speech to be effective at its base level, and thus Parma would as well. My player Bjrnaer has silent and subtle casting, so will be able to raise Parma while in bear form. But without those, or similar, Virtues, he would not. YMMV
Just like an apprentice cannot learn a spell by watching his master gesture and speak, so too he cannot learn to raise the Parma by watching his master conduct the ritual. In both cases there is definitely an internal part.
Notice that these virtues do not (IMO) affect Ritual spells. So... is the parma more like a ritual spell or a formulaic one? YMMV.