To trace a single contour, first make sure that "Tube:" is set to the correct name for the contour, then click either "TRACE CONTOUR" or (entirely equivalently) click anywhere on the image.
Skandha Morpho provides a number of ways of setting "Tube:" to the desired value.
The most straightforward is to click on "Tube:" and type in the desired name.
Once you get past the first frame of the day, Skandha Morpho will frequently bring up the correct "Tube:" name automatically. (In these cases, it will also prompt you with a crosshairs, indicating the size and position of that contour on the previous image, which is presumably approximately its size and position on the current image.)
If the contour name you want is displayed in the "DONE THIS FRAME" or "LEFT THIS FRAME" boxes (these appear only after you have digitized some contours), you may click on the name there to make it the current "Tube:" selection.
Once you have "Tube:" set correctly, click either on "TRACE CONTOUR", or else anywhere on the image. Skandha Morpho will prompt you to enter the contour, either by clicking one for each point, and twice to end ("Click-Per-Point" digitizing) or else by clicking once to start and once to stop ("Stream Tracing" digitizing).
If you are unsatisfied with a contour when done, Skandha Morpho currently provides no real editing facilities. You may, however, remeasure it completely by selecting its name from "DONE THIS FRAME" and then retracing.
When you have measured all desired contours on one frame, you may proceed to the next by selecting "Next Frame".