I would think that sewing a mask would be pretty simple. Pretty self explanatory. A rectangular face piece with or without darts for fitting and four tails of either sewn straps or woven cotton tape. The thickness or number of layers of the face piece could be varied according to the material used, degree of filtration desired and an acceptable level of resistance to the breath. Antibacterial treated cloth may be an option.

Plugging the terms: "sewing pattern surgical mask" into Google got me a few useful results. The most promising of these:

http://www.bevscountrycottage.com/hospital.html

http://www.lds.org/ldsfoundation/multimedia/files/pdf/51241_surgicalmask_pdf.pdf