As they stand, the current "skivvies" worn by the naked models are atrociously modern. The male model should have braies. Braies are basically baggy shorts, and in all illuminations from the 12th century on are ALWAYS white. The pattern looks like this:
They're made of three pieces of fabric, each ~50" long and 30" wide, which makes the waistline 120" (10' or 3m) around! As you can see, the inseam of the legs is not sewn up. This allows it to fit inside chausses or hosen with a minimum of unsightly bulging on the inner leg. The red drawstring is used to either secure the chausses/hose, or to tie away the loose legs. To tie away the legs, take the front bottom corners (A) and wrap them between the legs and around to the tie (B). The blue string
This detail from the Maciejowski Bible shows a man wearing nothing but braies, with the legs tied up. Note the hip roll, that's not just the drawstring channel. Once the braies are on, your pull all the excess material up and under the drawstring. Not only does this give the unfitted braies a better fit, but it also gives men "hips", which I can tell you is something you definitely want to keep your belt on without having to buckle so tight that it digs in and hurts by dinner time. More than anything else, having the hips from wearing proper braies gave my medieval garb the right "hang" to actually look medieval. I can wear my belt loose enough that it has a V-shaped dip in the front, and it will still stay in place all day long.
In this detail, we can see what the braies looked like under chausses.
There is little known about women's undergarments. They may have worn braies as well to hold their hose up, or they may have just worn a full-length white linen undergown. The Egerton Genesis portrayal of the rape of Dinah shows knee-high hose tied with a strap just below the knee, so in at least that depiction, the woman likely wore nothing under her gowns. If the female model isn't to be in a white undergown while "naked", she should also be in braies and have a linen wrap around her breasts.
They're made of three pieces of fabric, each ~50" long and 30" wide, which makes the waistline 120" (10' or 3m) around! As you can see, the inseam of the legs is not sewn up. This allows it to fit inside chausses or hosen with a minimum of unsightly bulging on the inner leg. The red drawstring is used to either secure the chausses/hose, or to tie away the loose legs. To tie away the legs, take the front bottom corners (A) and wrap them between the legs and around to the tie (B). The blue string
This detail from the Maciejowski Bible shows a man wearing nothing but braies, with the legs tied up. Note the hip roll, that's not just the drawstring channel. Once the braies are on, your pull all the excess material up and under the drawstring. Not only does this give the unfitted braies a better fit, but it also gives men "hips", which I can tell you is something you definitely want to keep your belt on without having to buckle so tight that it digs in and hurts by dinner time. More than anything else, having the hips from wearing proper braies gave my medieval garb the right "hang" to actually look medieval. I can wear my belt loose enough that it has a V-shaped dip in the front, and it will still stay in place all day long.
In this detail, we can see what the braies looked like under chausses.
There is little known about women's undergarments. They may have worn braies as well to hold their hose up, or they may have just worn a full-length white linen undergown. The Egerton Genesis portrayal of the rape of Dinah shows knee-high hose tied with a strap just below the knee, so in at least that depiction, the woman likely wore nothing under her gowns. If the female model isn't to be in a white undergown while "naked", she should also be in braies and have a linen wrap around her breasts.
