The easiest way is to go from Tor to a website proxy like nonymouse.org or hidemyass.com.
If you want to chain http proxy after Tor, you should put http proxy settings in your browser, and then route all browser's traffic through Tor by either torify or iptables, which is not trivial.
Please refer to this: https://trac.torproject.org/projects/to ... arentProxy
So, the full scheme: Browser thinks it uses 'normal' proxy and knows nothing about Tor. And you catch all browser's traffic and transparently route it through Tor, like it is made for programs that don't support proxy.