Act as a senior web developer with 5+ years of experience in static site generation. Guide me step-by-step through setting up a blog using Gatsby and Markdown. Start by explaining how to initialize a new Gatsby project using [YOUR_PREFERRED_PACKAGE_MANAGER]. Then, detail the process of configuring Gatsby to parse Markdown files from a [SPECIFIC_DIRECTORY_NAME] directory, including necessary plugins like `gatsby-source-filesystem` and `gatsby-transformer-remark`. Finally, demonstrate how to create a template for blog posts that includes [CUSTOM_FRONTMATTER_FIELDS] like title, date, and tags. Provide code snippets for each step and explain best practices for organizing your project structure.