1 - place a breakpoint in your microflow where you want to start debugging (note that the debugger should be connected to the local environment)
2- trigger the starting point before your microflow (button or something)
3- then you will see that microflow is hold on at the breakpoint you have placed, look to the variables windows to see the object states and check the attributes on correctness/emptyness..etc
4 - continue to proceed with your debugging process
additional debugging stuff which i occasionly do is:
a) using custom with rollback on the point of the error with the latestError/stacktrace logging
b) using different loglevels to see whats going on under water in mendix
How to enable changes window?
Modeler - View-> Changes.