The easiest way to do it is by using a calculator using the square root button or taking a number the 1/2 power which is the equivalent of taking the square root. I'm assuming that is not an option for you.
The next best option is basically trial and error. For example, if we want to find the root of a number such as 29. We can look for squares that are near this value (i.e. 25 and 36). From that we know the square root of 29 must be between 5 and 6. We can then continue to narrow it down (i.e. 5.5) and find the square and find it as 30.25 so we know it's slightly lower than 5.5. Continue this process until you get a value that's accurate enough (usually 1 or 2 decimal places).