For testing purposes the command below will generate a self-signed certificate for the domain example.com. This self-signed will cause warning messages about the certificate but ideal for testing configuration locally. When deploying into production, you will need certificates generated for your site from a service such as Let’s Encrypt.
The command below creates a new certificate and key within a directory calls certs/. It sets the domain to example.com.
mkdir certs; cd certs; openssl req -nodes -new -x509 \ -keyout example-com.key -out example-com.crt \ -days 365 \ -subj '/CN=example.com/O=My Company Name LTD./C=US'; cd -