Are we really star-stuff from the interior of collapsing stars? Carl Sagan has said several times that we are "star-stuff". 

One instance can be found in Good Reads' Carl Sagan > Quotes > Quotable Quote:


  The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.


Question: Was most of my nitrogen really made in the interior of a star during its collapse? Was my calcium and iron made there as well, and not (for example) in an expanding shell after a supernova?
The straightforward answer is, "Yes, we are made of star stuff."

Some of it will be from the interior of collapsing stars, some will be from supernovas, some from normal everyday fusion, and some from other processes.

The answers from @HDE226868 and @RobJeffries on this question on where heavier elements come from gives good background, including this nugget:


  The split between r-process and s-process production of heavier than iron (peak) elements is about 50:50. ie They weren't mainly made in supernovae, which is a frequent, incorrect claim.


but of most importance is Rob's final point:


  The relative contributions of various sites to the r-process remains an unsettled matter. You could also read my answers on this topic in Physics Stack Exchange.


On following Rob's links I think this provides you with an excellent overall answer (and relative percentages)


  A more up-to-date visualisation of what goes on (produced by Jennifer Johnson) and which attempts to identify the sites (as a percentage) for each chemical element is shown below. It should be stressed that the details are still subject to a lot of model-dependent uncertainty.
  


Looking at C and N - the majority seems to be from dying low mass stars, and Ca and Fe are from exploding stars, which indicates that Carl is not far off the mark.