When I read a textbook, a specification or an article or something technical that I consider important parts I use the Feynman Technique[0].

As from what the linked article says,

    The technique is simple:

        1. Get a piece of paper
        2. Write at the top the idea or process you want to understand
        3. Explain the idea, as if you were teaching it to someone else

    What’s crucial is that the third step will likely repeat some areas of the idea you already understand. However, eventually you’ll reach a stopping point where you can’t explain. That’s the precise gap in your understanding that you need to fill.

I used to repeatedly explain the idea to myself. At first I find it hard to recall though I don't feel discourage when I'm unable to hold it on my mind at first and just think that there are a few that can but once I train myself I'll eventually improve.

When reading blogs, HN comments, novels etc., I usually read it once just like I'm watching television or a movie.

I don't worry that I read slow since when I've found out that Donald Knuth also reads novels very slowly[1].

If you want a more systematic reading, there is a book[2] which has been mentioned often here in HN.

> Is there an app that you use?

I use org-mode for everything

[0] - http://calnewport.com/blog/2012/10/26/mastering-linear-algeb...

[1] - http://www-cs-faculty.stanford.edu/~uno/retd.html

[2] - https://www.amazon.com/How-Read-Book-Intelligent-Touchstone/...