To create a new certificate, execute the following OpenSSL command:
  (umask 077; \
  /usr/bin/openssl req -new -x509 -days 365 -nodes \
    -config /usr/share/doc/packages/stunnel/openssl.cnf \
    -out /etc/stunnel/stunnel.pem -keyout /etc/stunnel/stunnel.pem)
 
Starting with v4, stunnel uses a configuration file. 
Via the stunnel3 wrapper you can run stunnel with v3-style cmdline arguments.