Editing Creating a Self-Signed Certificate (section)

== Standard Certificates ==

You can do that in one command:

 openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365 -nodes

The '''-nodes''' option disables protecting your cert with a passphrase, otherwise it will prompt you for "at least a 4 character" password. The days parameter (365) you can replace with any number to affect expiration date. It will then prompt you for things like "Country Name" but you can just hit enter and accept defaults.
Self-signed certs are not validated with any third party unless you import them to the browsers previously. If you need more security, you should use a certificate signed by a CA

Add '''-subj '/CN=localhost'''' to suppress questions about the contents of the certificate (replace localhost with your desired domain)