There are a number of methods. The most common and most useful for small businesses is the Multiple of Discretionary Earnings method. Although there isn't room here for all the math, basically what you do is weight the 8 important characteristics, rate them each between 0 - 4 (4 is the highest rating) and then apply that weighted average to the Discretionary Earnings. Most small businesses will value out between 2x - 3.5x of Discretionary earnings. This method becomes less useful as the discretionary earnings increase. I wouldn't use this method for Discretionary Earnings in excess of $200,000.
Obviously you need to know how to determine the accurate Discretionary Earnings and how to measure the 8 critical factors in order for this method to be useful.