Alex Marquardt provides a detailed guide on how to debug Elasticsearch source code using IntelliJ IDEA, specifically for version 6.6. The process involves downloading the Elasticsearch source code from GitHub, configuring it with the Gradle build system, and setting up the Java environment variables necessary for building the project. The guide outlines steps for importing the Elasticsearch project into IntelliJ IDEA and starting the project in debug mode, which allows users to set breakpoints and step through the Elasticsearch and Lucene code. This setup is designed to help developers understand the inner workings of Elasticsearch, though it is noted that the instructions are specific to older versions and will not work for version 7.5 and later due to changes in the codebase. The blog post emphasizes the utility of reviewing accompanying text files like CONTRIBUTING.md and TESTING.asciidoc for additional guidance and encourages users to consult IntelliJ IDEA documentation for debugging assistance.