A Homemade mask is not considered true PPE and will not be as effective as using a medical grade, disposable mask. The CDC recommends all masks should cover the entire front (including below the chin) and sides of face.
The CDC has considered homemade face mask to be used a last resort in hospitals and medical facilities. Mask made from cotton and elastic certainly won't harm civilians at home or when going out, as long as you're also following other protocols to stay safe.
The use of face mask is not intended to replace other recommended measures to stop the community spread of Covid-19. Such as social distancing, washing your hands and refraining from touching your face. Follow the latest advice of the CDC and your own health care professionals as to how best to keep yourself safe.