Find a calculator or a pencil and paper.
Ask your friend or eveyone to write down their birthday.
Example :  September 28, 1986

Ask your friend (or everyone in the room) to write down the number of the month he/she/they were born.
Example : 9  (born in September)

Multiply the month by 4
9 x 4 = 36

Add 13
36 + 13 =  49

Multiply by 25
49  x  25 = 1225

Subtract 200
1225 - 200 = 1025

Add the day of the month he/she/they were born
1025 + 28 = 1053

Multiply by 2
1053 x 2 = 2106

Subtract 40
2106 - 40 = 2066

Multiply by 50
2066 x 50 = 103300

Add the last two digits of your year of birth (1986)
103300 + 86 = 103386

Ask your friend to give you their result
103386

Now you can magically tell them their birth date.
Here's how to do it:

Subtract 10500 from their result
103386 - 10500 = 92886
9 = month of September
28 = day of birth
86 = year of birth

You may now apply for your soothsayer licence!