$ useradd newuser

$ ssh-keygen -b 2048 -t rsa -f key -C newuser

$ su - newuser

$ mkdir .ssh && cd .ssh

$ cat > authorized_keys < key.pub

$ chmod 0700 ~/.ssh
$ chmod 0600 ~/.ssh/authorized_keys

Rename the key to newuserkey.pem for a clearer file purpose

$ mv key newuserkey.pem

Then to access the server from anywhere:

$ ssh -i newuserkey.pem newuser@ec2_host_name