To create a self-signed SSL cerificate in file.crt with key in file.key, you can run:

openssl req -x509 -newkey rsa:1024 -keyout file.key -out file.crt -days 9999 -nodes

If you are creating an SSL certificate to use for a web virtual host via DomTool, then you should generate a single output file instead of separate .crt and .key files. For example:

openssl req -x509 -newkey rsa:1024 -keyout file.pem -out file.pem -days 9999 -nodes