Travelers must complete the online pre-boarding “Health Declaration Form" at the Nigeria International Travel Portal. Travelers must have a negative COVID-19 PCR test issued within 96 hours of their departure to Nigeria, and have uploaded their PCR test certificate to the online Travel Porta.
Children under the age of 2 are exempt from presenting a COVID-19 test.
Travelers with proof of recovery from the virus within the last 90 days prior to boarding. Proof of a recent positive viral test and a letter from your healthcare provider or a public health official stating that you were cleared to travel.
More info at the CDC site
Travelers who did not get tested, are advised to stay home and self-quarantine for 10 days after travel.