How can I accurately place landmarks on 2D CT and MRI images of the knee? I have used the Markups tool before, but I’m not sure if I did it correctly.
Could anyone guide me through the process of placing landmarks using 3D Slicer?
Thanks! 
If you are using the tools correctly, there is nothing else. Accuracy comes from you finding the right anatomical marker to put the landmark on.
But here are some tutorials from SlicerMorph and Markups.