I want to get an intuitive understanding of Heun's method and why it has order of consistency 2, i.e. the error lies is in $O(h^{3})$ if $h$ is the stepwidth., I have read it everywhere, my professor says it without proving it. All the resources I have seen so far on the web either omit the proof as trivial, and the only one that does the proof does it wrong. I am at a loss. Can someone explain it to me in as much detail as possible?