The term 'rubakha' applies to both a Russian men's shirt and women's underdress. It seems to have varied little between the late tenth century and the end of period, and forms of it are still worn as Russian folk costume today. A pattern for the shirt can be found as Folkwear pattern #114.
The rubakha is usually made out of linen, though silk was used by the richer classes after around 1200, when that fabric became available through trade with the East. The shirt reaches to about midthigh. I have seen pictures of it worn both hanging out and tucked in, usually with a belt or sash. The dress drops to the lower calf or ankle. Both are white or off-white; the men's shirt frequently has red piping at the seams and red underarm gussets.
Both shirt and dress use a shoulder yoke - typically square in the Ukrane, and triangular in Russia. The body is made from straight front and back pieces with side panels added in below the sleeves. These side panels widen towards the bottom in the dress. The sleeves themselves are straight and long. Those for the dress are gathered into a band at the cuff; for the shirt they are left straight. The neckline is round, and fairly close, with a thumb-high collar. It opens in a slit either down the front or slightly to one side to allow the head in. The slit can be closed with either ties or buttons.
Both shirt and dress are decorated with bands of embroidery at the cuffs, and around the collar and hem. Sometimes a band of embroidery also followed the collar slit. Russian women spent quite a lot of time on fantastically complex decorations, using embroidery in strong primary colors, gold and silver thread, semiprecious stones, and copious amounts of freshwater pearls. However, we in the SCA usually don't have that kind of time; bought trim works just as well, and you can enhance it by adding pearls in strategic places. Floral patterns were common, as were geometrics. There are several books of typical Russian patterns.
Medvedeva, Yelizaveta. Class given at Pennsic XXVI on Russian Garb. August 13, 1997.
Rostrovskogo, Mordak and Anastasia Ivanova. Class given at Pennsic XXVI on Russian Costume Decoration and Accessories. August 13, 1997.
Soraya Evodia of Edessa. "An Introduction to Russian Costume." Compleat Anachronist, No. 35, Milpitas, CA, Society for Creative Anachronism, 1987.
Yarwood, Doreen. The Encyclopedia of World Costume, New York, Scribner, 1978. Pp. 348-350.