We provide a tutorial on how to create link budgets and bit error rate (BER) and probability of fade calculations for optical communications systems designed to operate in the turbulent channel. It reviews the characterization models necessary for either incoherent or coherent free space optical communications uplink, downlink, and horizontal system analyses in the turbulent channel. Beam wander, scintillation, and receiver noise variance as well as pointing and tracking effects were included in this paper. Comparisons among these models, computer simulations and field measurements are provided throughout the paper. Good agreement is shown among all. An example analysis was provided using this information. The conclusion is that no matter whether the system is incoherent or coherent and/or which signaling format is used, the scintillation index will peg the BER to constant value at high signal-to-noise ratio well above the desired value. Other turbulence mitigation techniques must be employed to get the desired system performance.