>> how does it work?

Your essence card is one of the 22 major Arcana. To find it, add the digits of your date of birth. In my case (24.12.1984) this would be 2+4+1+2+1+9+8+4=31. Since the highest number of the major Arcana is 21, the World, I add the digits again and find the next cross sum: 3+1=4. My essence card is the card number 4 (Emperor).
Should the cross sum of your date of birth give you a number below 22 but above 10 this means that there are two cards that could be your essence card. For instance 18 (Moon) and 1+8=9 (Hermit). There are several ways to read such dual essence cards. You can see them both as equal - both are essence cards. You can see one card as describing your essence, the other as describing your outer demeanour. I personally prefer to regard the card with the higher cross sum as a chance or a quest that lies in the essence card (the one with the lower cross sum). Let's say someone has the Moon as a "quest" from the essence card Hermit. This could mean that this person should take special care that their need for solitude and for introversy (Hermit) does not change into a prevailing mood of depression (Moon).