A sidetrack assembly for forming a lateral wellbore includes a cutting device having a body having a passage, and a mill head configured to connect to the body so that an interior of the mill head and the body form a pressure chamber in fluid communication with the outlet passage. The mill head includes a first port in fluid communication with the pressure chamber, a second port in fluid communication with the pressure chamber, and a blade arranged on a face of the mill head. A whipstock connected to the cutting device includes a tubing having a first end removably inserted in the first port and the passage and a second end connectable to a downhole tool, an inclined surface for guiding the mill at a non-zero angle relative to a central axis of an existing wellbore, and an attachment section removably connecting the cutting device and the whipstock.