For clarification purposes only, I teach the garments by a "standard name set" so confusion does not set in. This of course do not allow for the variations that did occur in period where the names for garments were mixed and blended. A cotehardie in France was a gown in England, robe was the whole outfit in some areas. Kirtle (guarnica in Italy) has been consistently a name for an "underneath dress" off and on through out period. There are cases in the 12th century, though that it was the name for an overdress.
There are so many posts to answer that I'll start underneath and work my way to the outside.
The first layer (optional unless doing short sleeved kirtle) is a chemise (best seen in the Illustration of June in John, Duc du Berry's Tres Riches Heures). This is a simple T-tunic cut dress with slight flare from the armpit. I have never found a need to gusset armpits (I know someone will ask that question). There is a version, seen in the Wenceslas Bible, which is sleeveless. I would forgo this in warm climates if wearing a long-sleeved Kirtle...
This is the foundation garment layer. If you make this correctly, you will not need a bra! I mean this for all body types, because I have done it (made them) and seen it done successfully (by others)! Calontir is chock full of ample women wearing kirtles and cotes! (This is probably my biggest pet peeve: the people who just dismiss this patterning and go to princess seams and a bra because they think they are "too big" to be supported.)
The most common 14th C. version starts as a 4 panel dress with either long sleeves that button or not, short sleeves, or no sleeves. (This can also be done using the Hjerolfnes dress pattern, but that is a task for more experienced clothiers.) Think 4 panel t-tunic dress that you will now begin to tailor to your shape.
Here are the measurements you need to take. You should always start with the selvage as your center seam. This provides non-stretchable support to your breasts!Bust
When marking the fabric, start measuring down from the bust line._______________________ <-Edge of fabric
For the neck, I would cut it like a t-tunic at first, then alter when you are fitting your body. My rule of thumb is to work from the fabric corner, go 3 inches across and 3.5 down for a circle. Always go down more than you go across (keep from having boat necks). If you need to make it bigger to start, just use the more down rule. You can always make the hole bigger, not smaller!
Granted, I have shown you a woman with "standard/straight" proportions here, 36, 29, 40 (me). If your waist is not the same measure as your underbust, just flair from the underbust point. You will get all your bust support, and not of the tight ribs. This is perfect for earth mother types and maternity versions of the kirtle as well!
Now, join all measurements and flair from the hip.
For the armhole, trace a simple curve from the end of the shoulder measurement down to the end of the bust measurement. To verify how deep it should curve, you can measure from the center of the breast bone to the crease of where your arm meets your body.
Make 4 of these. To do my test pattern, I make crappy fabric mockups that go to the bottom of my butt. I sew them together with basting stitches and get a friend to help fit. Anyone can help. Fun for spouses or significant others! Remember you will need to pick up your breasts and drop them into place in the shelf. If that isn't happening, then your mock is too loose. Think of this as fitting like a push-up bra!
The places you want to fit are pinching in the side seams, the back seam, and pulling up on the shoulder seams. DO NOT ALTER THE CENTER FRONT SEAM IN ANY WAY! Pin as you go and trim the neckline and armpits last. If the armpits will pook a bit, don't worry, this goes away when the sleeves are put on. If you get a wrinkle in the waist due to a sharp curve in the Lumbar spine, ease out the waist a touch, or curve in the back seam. This wrinkle is probably period, just like on hosen with the feet cut as one with the leg... (but I digress).
When finished pinning, this will be tight enough that you won't be able to get out of it by pulling it over your head. (Actually, that should have been slightly difficult when you put it on...) Just rip the basting stitches in the center front seam to get out! Someone once used the phrase "Set those puppies free!"
I line just the upper body of my garments, all the way down the center front opening. I also do my lacings (or buttons for the cote) past my navel so I can just drop them when unlaced or unbuttoned - makes a great "romance novel moment" :P Then when you get undressed at an event, jeans go on first, you unbutton the cote, leave it around your hips while unlacing the kirtle, and then drop them both when you get your bra and shirt on. Fold both up one inside the other and off you go!
For lacings, you can do the eyelets by hand, using an awl and a screwdriver (modern sewing awls don't make big enough holes) and finishing with embroidery floss or use a very small buttonhole on your machine (I do that as a quick for non-competition stuff).
Use the exact same pattern for the cotehardie, or overdress, (I just call them cotes like much of the literature of period). I just cut it a bit bigger (1/2 inch on each side).
If you want to do buttons all the way down the front, add about 3/4 to 1 inch to each front panel so you have overlap.
I hope all this helps.