The objective of this work is to develop an efficient method to carry out the preliminary design of the mooring system for a wave energy converter. A practical mooring design procedure is applied to a specific case of study, and it can be replicated for other cases. Firstly, the static analysis is performed for a configuration with three mooring cables with different pre-tensions on fairlead, diameters of the cables, and materials. Based on these configurations from the static analysis, a quasi-static analysis is carried out in the frequency domain and a preliminary design is conducted according to DNV rules. Then, a 3-h dynamic analysis in the time domain is performed on several selected configurations, considering the same environmental conditions in the quasi-static analysis using the finite element method. Extreme dynamic responses of the system, such as extreme surge motion and mooring tensions, are estimated by the global maximum method, which is performed by fitting 20 individual maximum observations by Gumbel distribution. The quasi-static method is validated by comparing the results of extreme tension and displacement with the coupled time domain analysis. In addition, the influence of pre-tensions and cable diameters on the static and dynamic responses of the mooring system are discussed.