Create Jekyll redirect links when migrating your website, for links that changed or that were removed.

Add this to the Gemfile in the plugins block:

gem 'jekyll-redirect-from', github: 'jekyll/jekyll-redirect-from'

Then run bundle install.

As seen in the docs here

Add it to the _config.yml under plugins

- jekyll-redirect-from

My problem was that for my previous website, the blog posts had this format:

blog_post_name.html

This new website removes the .html and a lot of blog posts crawled by google are being sent to a 404 File not found page.

I want to redirect blog_post_name.html to blog_post_name/

In the blog post the header should show something like this:

title: "Blog Post Name"
redirect_from:
  - /blog_post_name.html

404 page

More about 404 pages here

You can create a 404.md file if you add this:

---
layout: page
title: "Not Found"
permalink: "/404.html"
comments: false
redirect_from:
  - /index2.html
---

Use the redirect_from: to redirect bad URLs from Google search results. Or use the same approach for blog posts. You can also try to fix them in your Google Search Console.