Women’s 15th century Smock or Shift
This is your underwear, it is always made of bleached white linen.
It should have long sleeves, under-arm gussets, a scoop neck and be between lower calf and ankle in length. The sleeves need to fit under the fitted sleeves of the gowns worn over it. There are no drawstrings on the garment at all nor any gathering anywhere. You want a smooth fitted finish at the neckline, bust and shoulders to prevent bunching up that can become uncomfortable. The scoop neck should be low enough to not stick out much past the necklines of the gowns worn over it (a peek of your smock in the neckline of your gown is fine but ideally it should not stick out at all).
A possibility to consider while making your shift is to leave the neckline unfinished and cut deliberately too high until after your laced front kirtle gown is completed. Then you may use the neckline of that gown to mark your shift neckline exactly (while wearing the gown over it). The shift may or may not (your choice) be cut with gores inserted in the side seams from the hip to hem. You can machine sew your shift and finish all visible areas (neckline, hems & wrists) by hand.