Right click with mouse to the left of code to add/remove breakpoints.
All gdb commands work from gdb window at bottom of screen.
When program crashes, first look at stack back-trace,by choosing <menu> → Status → Backtrace.
To display data structures use display section, right-click on a variable and choose display.
In display section, double click on a pointer to dereference it.
To have DDD detect the same object is being referenced from two places, type Alt-a. Or <menu> → Data → Detect Aliases. In the latest version, this is default behaviour.
When debugging C++, to avoid stepping into STL functions, one should normally use Next; use Step when you specifically want to step into a function.