How may I teach myself pure mathematics "from scratch"?
Look at undergrad math curricula of some good universities - that should tell you the order of math subjects that's typically used to teach undergrad students.
Then study the subjects one by one. You can look up good books for each subject on Amazon going by customer reviews. Or you can search for a reference request question for that subject on SE.
Download the PDF of a good book if available; you can also read course notes for those subjects on college/university websites just so you know what topics are typically covered in undergrad courses.
For example, a linear algebra book will usually contain much more than what's taught in a first-year undergrad linear algebra course, so referring to course materials can help you keep track of what to cover in a first course/reading.
Hope that helps!
You could look for videos on Coursera and on YouTube. MIT OpenCourseWare is pretty good and free on YouTube. They will advice you the books.