The following snippet adds a forward proxy to an apache web server in a VirtualHost listening on port 9090.
To use, configure your browser so as to use your apache server as a proxy on port 9090

# Forward proxy
Listen 9090
<VirtualHost _default_:9090>

        #--- logs configuration
        ErrorLog /var/log/apache2/forward-proxy.log
        TransferLog /var/log/apache2/forward-proxy.log
        LogLevel info

        ProxyRequests On
        ProxyVia Full

        <Proxy *>
          # if using Apache 2.2 use Order, Allow directives
          Order deny,allow
          Allow from all
          # if using Apache 2.4 use Require directive
           Require all granted