I first used it about 25 years ago when there was an elaborate sequence of steps to do. It has been greatly simplified today.

I haven't used it lately as I have other tools, but it did work, especially when I had overwhelming depression/pain well up inside me. I would tap furiously while I sat and felt the pain. I'm talking pain that could bring me to my knees. Really feel it (but don't engage with it), as strongly as you dare. Don't block it or suppress it in any way while you tap on your chosen "site".

Eventually, the pain will dissipate, and you will feel incredible relief.
Sometimes, you need to repeat the above to get the full release of your emotional pain or issue.

There are quite a few books on it, but you can find all you need on internet sites.

PS: By "don't engage" I mean don't relate your emotional pain to any event. It is in you, you want it out.