Be sure to visit the Trace Modeler website for updates and UML sequence diagram resources!
Trace Modeler automatically determines at which a particular target a message should originate, based on the control flow.
This means you can only change the sender of a message by influencing the control flow. You change the control flow by reordering messages :
It is actually a lot easier than you would expect from the above. Just experiment with try moving messages around and you'll quickly get the hang of it. When starting out, it might be easier to try this with the control flow highlighted.