Long story short, these steps would help you out if you want to solve the same problem:
1. Go to "Template", then click "Edit HTML"
2. Insert this piece of code between the tags <head>...</head>
Note: To avoid problems, it is suggested to insert this just above the closing tag </head>. And be sure that the indentation is logical, i.e., the inserted code is inside the <head>...</head> tags.
3. Save temple, refresh Blogger.
That's it!
Next you can test with an example: first, create a new post, fill it with an equations in LaTex format. Then click preview, you should see the equation appears properly in your new post. (It might take 3 seconds to appear, be patient).
Here is my example: $ E=mc^2 $
There is a lot other ways to make this work. But I found out this is the simplest way for me.
