Witchipedia

There are many different methods within different traditions. I personally do a lot of dream work, astral travel and medium type work and I just listened closely to how I was addressed by the entities that I spoke to, they had a name for me. It didn't translate well into the spoken word so I searched around through various name dictionaries until I found one that meant the same thing and that became my magical name.

Another method that seems popular is to use numerology. There are a few different methods, this article gives a good overview and instruction http://pagannameswiki.wikidot.com/article:using-numerology-to-choose-a-name

Either way, you don't need to rush into it. And it doesn't need to be your name forever. Many people choose a new name when they reach different points in their lives. You know in the Catholic tradition you get a new name when you're baptized, and many take on a new name when they're married, and there are many occasions when you may take on a new title that then becomes part of your name (Like Sir, Doctor, Professor, etc.). So it's possible that whatever name you choose now, you may decide is not appropriate when you move to the next phase of your practice and that's okay. You may also join a Coven one day that assigns new names upon initiation, that's cool too. You may combine names or leave an old name behind as a symbol of your moving to a new phase in your life. You may, as I do, have a public name and a private name-shared only with those you work with.

It took me a long time to find my name, two years to be exact. But when I found it I could feel it in my bones, I am a Kitchen Witch and I pick up on certain energies very well so that probably played a part, but none the less when you do find your name you will know and it will seem to call out to you