The proportion of oil to flour is 1 to 1. (1 cup flour and 1 cup oil). The color of the a dark roux should be that of cocoa, or hot chocolate.
To start, use a large heavy grade sauce pan. I mean a really thick pan.
Heat the oil to smoking point.
Now add an equal amount of flour, whisking quickly to avoid burning the roux. Now turn flame to medium, and continue whisking. Cook roux until desired color is achieved.
Remove from heat, and place into a heat proof container. This will stop the browning process.
NOTE: Brown roux is extremely hot. Care should be taken not to spatter the roux on your hands.