Assuming you mean sqrt(n) where n is a composite number, you take the prime factorization of n, which will give you dome multiples and exponents. Whichever exponent is equal or higher to the index of the radical, you can take it outside of the radical by groups of the index.
Basically, if you have sqrt(125), you factorize 125 to sqrt(5^3), then you take 5^2 out because the index of the radical is 2, anf youre left with 5*sqrt(5)
Same with, for example, sqrt(12), which is sqrt(2^2 * 3), you can take out 2^2 and you get 2 * sqrt(3)