Although I finished my 1660s gown nicely on time in 2017, the outfit wasnt’t quite complete yet. Most importantly, I still needed a shift!
In many 1660’s portraits, you see large white ‘under’ sleeves beneath the bodice sleeves, which are (I suspect) usually the sleeves of the shift. Additionally, you often get a bit of white fabric above the neckline of the bodice. Again, probably usually the shift.
Although you see many different styles, both in neckline and undersleeve, this was the look I was going for.

Caspar Netscher The lady at the window (1666, Heydt Museum Wuppertal)
So a large pouf with a ruffle beneath the sleeve, and a thin white band above the neckline.
Just in comparison, some of the other styles I found.
Many Dutch portraits show the more ‘modest’ (protestant?) look with narrow sleeve cuffs and/or a large lace collar. I might try my hand at these as well some time, but for the ball I wanted a more ‘evening’ look.

Portret van een jonge vrouw, Isaack Luttichuys, 1656 – Rijksmuseum
You also sometimes see a clear ruffle above the neckline, instead of just a narrow band.

c. 1668 Elizabeth Wriothesley, Countess of Northumberland, later Countess of Montagu (1646-90) by Peter Lely
Additionally, some sleeves seem to be gathered up and pinned, instead of having the ‘pouf’. You also see some sheer fabric in the necklines, which is different from the sleeves. I suspect these are separate and draped on top, although I’m by no means an expert.

Margaretha Van Raephorst by Johannes Mijtens, 1668
Basically the main conjecture for shifts seems to be: gathered neckline, with or without extra ruffle, and long wide sleeves, either pinned up or gathered into a pouf. The portraits showing ladies in their underwear seem to confirm this.
Two portraits of Nell Gwyn. The first shows the gathered strip at the neckline.

Portrait of Nell Gwyn.
Painting by Sir Peter Lely
This second one is just gathered at the top.

Nell Gwyn (v 1680) by Simon Verelst (1644-1721)
In the end, I chose to base my shift on the one made by Before the Automobile. I liked her method of seamless gores, and it ticked also the boxes of having a pouf sleeve (although I made mine slightly longer) and a band at the neckline.
I sort-of measured the neckline to get the width of the finished shift, and cut my pieces about 2x as wide to allow for gathering. The shift is about 1m long from the neckline down. The eventual pattern pieces about these sizes: Front: 75 wide, 100cm long. Back: 75 wide, 105cm long, Sleeves: 60 wide, 50 long, Gussets: 13×13, Gores: 70 wide at the bottom, length to fit with front & back.
Before any gathering. The back is slightly higher than the front.
I sewed most of the shift by machine as I wanted to finish it in a day. I might go back and hand-finish the seams from the inside.
The sleeves were hemmed with a small hand-sewn hem-stitch though, as these will show underneath the dress. (I need to iron them I see…)
Some more pictures (apologies for the grainy quality, I won’t have any opportunities to take pictures with daylight for 2 weeks, so it was dark when I took these).
The gathering on the neckline and sleeve.
Left is the view from under the arm, from the side. The gusset is inserted into the gore, which is cut open, and then attached to the sleeve. And left is the finished shift!