The article Here’s what WHO says your mask should have to prevent COVID-19 spread by Beth Mole (Ars Technica) details the technical specifications for making your own facemask. tl;dr: “you’re probably doing it wrong, guidance suggests.”

The WHO says:

• “[…] masks should only ever be used as part of a comprehensive strategy in the fight against COVID.”
• “WHO now recommends that healthy members of the public wear homemade or commercially-available fabric masks in places where the new coronavirus is circulating widely and where physical distancing (staying 6-feet apart, etc.) is not possible or is difficult.”
• “[…] a minimum of three layers is required for fabric masks”
• “[…] these masks are for source control only, not personal protection—that is, they can help prevent the person wearing the mask from spreading the virus, but they will not necessarily protect the wearer from becoming infected.”
• “Masks are not a replacement for physical distancing, hand hygiene, and other public health measures,”