The simple answer is that you don't. The Duet Web Control is the normal way of interacting with the printer, and uploading the sliced gcode and printing it from there is the best way to go.
If you want to connect to the Duet via USB and use a terminal program, you still can do that, and you can even stream gcode over USB to print, but it's not recommended and misses the point of the Duet entirely.