This was a something a reader wanted to see more written about so i hope this helps!
Psi-Feeding is a process pretty much any living thing can do. Most people and even animals do it to a small extent naturally. If the energy of the room they walk into is really happy, they in turn will feel the same way pretty quickly. But of course we will be focusing on it more in depth than the basic.
The key to energy is actually mentioned by Tesla himself. Energy, life, the universe is all about vibrations and tones. Every type of energy has them, and can be affected by those based on outside vibrations and tones. It is what is most used in magic and energy work as well even if folks do not realize it. Perhaps with the right vibrations and tones would could change another even!
Now how might one use vibrations and tones to change things? Quite simple actually and is one thing taught in many things such as magic, law, psychology and various other things. Your voice and the tone and vibration of it has power. Learning how to harness it is the key here. Yes trying out different tones in your voice is where you start. Levels of volume, pitch, etc. Once you have worked on that some then picture your voice box empty, and see and feel energy going into it. Think of having a dial on that box that will control the energy and its vibrations that will come from it. Now pick a tone and put some energy into it, the vibration of your voice will change. Allowing you to affect others, the world, and reality itself if you are using it in spell casting. Of course it can take ages, maybe even longer than ones lifetime to figure out and witness how you can affect and change the vibrations of other things (There are a lot of things on this planet alone!) but it can be well worth it, and each thing learned is knowledge you can use.
Now how might one use this to psi feed? Quite interesting enough with enough practice you can see and sense the type of energy and vibrations you prefer/need to feed on the most. Be it surface, sexual, elemental, core, etc. type of feeds. Once you learn which you need, you can seek it out by vibration/feel alone. It can also help you separate other energies to get to what one you actually need the most.
How would one go about seeing/feeling the vibrations? Simply use the skills you have learned from yourself and your own voice and how that felt, then reach out and feel. You will find many vibrations of many things ping back against you. This can of course be quite overwhelming for a while until you adjust to such a thing. Once you do that you will need to feed on someone or something. Watch and feel the vibrations that resonate the best with you. That will be the one you want to go for from now on. I hope this helps!