Create X509 Cert Using OpenSSL on Mac OS X

In a terminal, execute:

openssl req -out csr.csr -new -newkey rsa:2048 -nodes -keyout privatekey.key

Then:

openssl req -x509 -sha256 -nodes -days 365 -newkey rsa:2048 -keyout privatekey.key -out certificate.crt

See here