The CDC offers a series of guidelines, here a summary:
As masks are becoming difficult to find, there has been a high request for DIY masks, below a list of few. Many videos are also available
In regards to DIY masks, the CDC reports that:
Healthcare Personnel use of homemade masks:
"In settings where facemasks are not available, Healthcare personnel (HCP) might use homemade masks (e.g., bandana, scarf) for care of patients with COVID-19 as a last resort. However, homemade masks are not considered personal protection equipment (PPE), since their capability to protect HCP is unknown. Caution should be exercised when considering this option. Homemade masks should ideally be used in combination with a face shield that covers the entire front (that extends to the chin or below) and sides of the face."