Take a look at John Hull 8ed, p266
If you set the mean and variance of the binomial tree equal to that of the Geometric Brownian motion, you have two equations so that you can solve it, but maybe it need some approximation which is mentioned in the footnote.
For more clear derivation, see CRR's original paper "Option Pricing: A Simplified Approach"(1979)
Here is the a picture from Hull's book,
best,