OpenSSL: How To Create Self-Signed Certificate
$ openssl genrsa -out ca.key 1024
$ openssl req -new -key ca.key -out ca.csr
$ openssl x509 -req -days 365 -in ca.csr -signkey ca.key -out ca.crt
For the explanation, here’s the source: OpenSSL: How To Create Self-Signed Certificate