Official AWS documentation
Allocation:
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip-addresses-eip.html#using-instance-addressing-eips-allocating
Association:
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip-addresses-eip.html#using-instance-addressing-eips-associating
Log in to your aws account and look for the ec2 service
From the ec2 menu go to Network & Security and click on Elastic IPs
From the elastic ips menu click on Allocate Elastic IP Address
Lastly, just click on Allocate
Now select your ip and click on the actions buttons, then click on Associate Elastic IP address
Finally, select your ec2 instance id and its private ip address. Then click on Associate.