The Birka style of kaftan is basically just a tunic with a cut all the way up the front, plus an optional back split.
The back is one piece, like a normal tunic. I guess you could make it in two pieces if you really fancied, but there's no evidence so why would you?
You could make it without side gores (much like you could make a Norman style front & rear split tunic without side gores) but personal experience says they're not as nice to wear, don't move as well, are too tight.
Shoulders, arms and underarms are just like standard tunics.
In conclusion, use the standard tunic pattern.

The standing collar is based entirely on one surviving find (pictured here). That one is 51 cm long. Which is a good 20 inch diameter neck hole. It's hard to tell, but it doesn't seem to have been made to overlap like a modern shirt and doesn't seem to have had any sort of clasp? Most people reconstruct it to be standing and not overlapping, but possibly butting. Like a Chinese shirt, basically- look up Mandarin collars or Nehru collars.

Buttons. Kaftans are buttoned from the neck to the waist, no lower.
You could just attach the buttons to one edge of the kaftan, and the loops to the other? But that wouldn't show them off as well, so most people instead go for frogging. Frogging is, again, like you see on Chinese shirts. But keep the fancy big swirls to a minimum, you just want straight horizontal lines.
Basically get two bits of cord the same length. Thread a button onto the middle of one. Then double them over, and sew the two cords onto the surface of your Kaftan. One has the button attached, the other has a loop at the end you use as the buttonhole. Success!
If you don't have authenti-buttons, here are instructions on making pretty knots to use as buttons on the second half of this page (ignore the big swirl loops on the first half!)

Anyway, I don't think I'll be using real metal thread. Instead I'll probably go for embroidery silk. Is this going to be available at the next NLHF in February? The name Debbie Bliss does not appear on the trader's list. Doing a search on the internet, I found this:
http://www.angelyarns.com/bliss/debbie-bliss.php/yarn/111
Is this the right sort of thing? It doesn't mention anything about how loosely it is spun though.

As an aside: I haven't made decisions on colour yet, it'll depend also on the silk that'll form the basis for the embroidery. My kaftan is a dark green with a slightly lighter green lining and I'm planning to make the button loops from blue wool, so I'll probably also go for something blueish for the silk, which will go on the cuffs and around the neck and then probably a silvery/metal-like embroidery, but I'm not sure yet.

The ultimate authentic pattern to copy is the one Peter Beatson at Miklagard/New Varangian used. The original find is 10th century Swedish, lovely swirly patterns embroidered onto red silk using silver thread, used to make big cuffs and a standing collar. Mmm… You can definitely buy silvery metal thread that will do as silver substitute- I have some. Haven't tried to use it yet tho.

Two Kaftans is not at all excessive. It is very good, in fact. It's just a shame that you can't wear them at the same time (although I'm sure that won't stop you from trying).

Anyway, any ideas on how best to go about silk cuffs/neck? There are some examples on the pages linked from the kit section, but I'm not sure how to make them. Normal embroidery - if so, with what material? If not, how? I'm sure I can find a decent authentic pattern for it, but I'm wondering about the actual techniques. I guess I'll go off and do more research!

I haven't seen any evidence for full-length coats beyond knee length. Tunic/knee length, yes. Longer? Not that I've seen. Some nomadic coats are similar. They just have one or two frogs, at belt-length. They can then be covered by a big wide silk belt.

On the other hand, I haven't read the Hagg 1986 Birka article that talked about buttonless. My gut feeling is that it'll be kaftans without surviving buttons, as in ones held by wooden toggles or cloth frogs or similar. The famous Valsgaarde find with the pretty embroidered stand-up collar was from a grave without buttons, but again that might have been perishable buttons.

I'm definitely making two, one un-fur-lined and one fur-lined. The unlined will be asymetrical, not yet certain about the lined…

Bunni - are you making a nomadic kaftan AND a furry Rus one? Thats a lot of kaftans…(which is, of course, very good indeed.)

I haven't yet decided how to decorate the sleeves and edges though. Some embroidered (?) silk bands would be nice…

I've been looking through my new nomadic book, and they do feature a couple of long buttonless coats, held together by wide cloth belts. These are on the more central asian nomads tho (rather than the western nomads we want), so less applicable for our kit. Still, thought I'd mention it.

For my kaftans, I'm still undecided between using wooden toggles or thread frogs. I've found some nice frogs, and they will look more fancy…
Think I'm going to go for the off-centre look tho.

Now as I might have mentioned, I'm planning to line it with another layer of wool (I have two rather nice pieces of really quite thin wool, so it wouldn't be too thick in the end). I'm not quite sure about the authenticity however. I haven't come across any mention of thin wool lining, although in general lining seems to have not been unusual. It wouldn't exactly seem irrational to have two layers of wool given the climate in some of the Viking/Rus areas. But if anyone has relatively conclusive evidence (as far as that's possible) that that's definately something that didn't happen, then let me know before it's too late!

I'd want to read it to find out if they mean kaftans made without buttons, or kaftans without surviving buttons. Are they just cloth or wooden buttons that haven't survived the centuries?

The good link is http://www.miklagard.nvg.org.au/costume/rus/trader/rus_description.htm
Steer clear of making a hidiously hugely gussetted one like the ShadowDragon. You *can* make a non-rear-split riding coat, and it should be quite gussetted, but not that much. That's much too much.

I'll be using wool and also wool to line it (two different shades of green), but the material is quite thin, so it should nevertheless be comparatively light weight. I haven't started worrying about silk cuffs or fur around the edges yet. … Yet.