If you have an existing parcel/field that you would like to add features to please select this first.
Select Draw - Line
If drawing a linear feature that needs to snap to boundary/other feature the snap feature is on by default and will snap to that feature/boundary as you draw. It's a single click to start to draw and double click to finish.
With the line selected you can then assign a use if needed. Click on Assign Use and then search for the required option
You can also set the length of a line and this guidance shows how.