Simply start with a square (or round) wire and use the mill in the same way as any other manual mill. Square wire will gradually compress to round as it passes through consecutively smaller grooves. Note that a round section will be formed before the rollers are fully closed. If you continue to roll and fully close the rollers, this will create a slightly oval wire.
To avoid flashing, rotate the wire 90 degrees on every pass. If flashing forms, reduce the amount of reduction on each pass.