You may have been told to write the introduction to your essay last, but this piece of advice ignores the important role writing an introduction will probably have in defining the problem writing the essay represents for you. An introduction is the place where you can consider the rhetorical situation of your essay, and your own position as a writer and so build in your reflective practice to the essay structure. It will be much easier for you to give appropriate signposting in the course of your argument if you have already worked on the introduction.
What this advice to write the introduction last really means is that you can improve the effectiveness of your essay in the editing phase by thoroughly revising, or perhaps even rewriting, the version of the introduction you originally came up with, since the introduction plays a crucial role in framing the reader's experience of your essay. From this point of view, thinking about your introduction as something you're going to come back to after you've written the rest of the essay, with the benefit of a clearer mental picture of the essay as a whole, can be one of the most effective strategies against writer's block, given that you will probably find starting to put words down on paper one of the hardest parts of the writing process.