>>7636
>By concentrating on a single thought I mean something like visualizing a red triangle or contemplating the nature of the sun (what it does, how it works, etc).
This doesn't sound different from working with the mind, where you keep an uninterrupted flow of thinking on-topic for solving difficult problems. If I need to do this I usually go lie down on the kitchen sofa with my hands behind my back, feet on the armstand and eyes closed. I used to practice self-hypnosis so I easily get into a deeper state this way.
I used this to learn it:
https://theartofhypnosis.blogspot.com/2013/09/the-art-of-covert-hypnosis-by-steven.html
There is a link for the book in the comment section.

>if you want to be able to do "physical magic" one day. By "physical magic", I mean magic that can cause actual changes to physical reality, magic that you can test and observe with your physical senses
From your later descriptions it's not what I practice, but for just effecting changes you don't need this ability. I do conventional spell casting or summoning and this works fine without that specific ability of focusing on a single image. My visualization is mediocre at best, which is why I use words instead. Energy projection or manipulating events and people doesn't require the kind of focus you are training for.

>Bruce Frantzis
Never heard of him.

>felt energy before from qi gong
My body turned very hot the first time I tried a free session held by a local teacher, it was the Biyun Qigong. It's a "medical qi gong" authorized by the Chinese Government which was pushed a bit 20 years ago. I don't know what you mean by "feeling energy", I can effect my own body and other people, including mental states and manifesting the effect of different drugs temporarily, as well as flush through with energy to open energy streams using an external force. I never attempt healing because of its dangers, I refer to it as removing astral parasites or opening energy channels from outside.

I don't really care about trying to feel the energy itself, though I've always had an ability to see it as colours, like an Augmented Reality overlay on my vision. For more complex work I close my eyes and "pull in" images into my inner vision instead of looking with the eyes.