Here's an example it's called the "divide and average" method
Sqrt 13 (to three decimal places) = 3.605
First since the sqrt 9 is 3 and the square root of 16 is 4......let's guess that the sqrt 13 might be something like 3.5 ( actually......we can make any "educated guess"....it will still work out
1) Divide 13 by 3.5 = 3.174
2) Take the average of 3.5 and 3174 = 3.337
3) Divide 13 by 3.337 = 3.895
4) Take the average of 3.337 and 3.895 = 3.632
5) Divide 13 by 3.632 = 3.579
6) Average 3.632 and 3.579 = 3.605
And our job is complete !!!!........hope that might help you !!!