It's actually quite easy, assuming you have an openssl installation handy. (What platform are you on?)
Assuming you're on linux/solaris/mac os/x, Van's Apache SSL/TLS mini-HOWTO has an excellent walkthrough that I won't reproduce here.
However, the executive summary is that you have to create a self-signed certificate. Since you're running apache for localhost presumably for development (i.e. not a public web server), you'll know that you can trust the self-signed certificate and can ignore the warnings that your browser will throw at you.