In order to perform an equivalent static analysis, one has to follow a code. The code provides you with some basic informations like- Elastic or inelastic response spectra, some empirical formula to compute the approximate fundamental time period of the structure. The approximate fundamental time period of the structure represents the fundamental mode where the mass participation is expected to be the highest (generally 1st mode of vibration). Generally, one need to use the equivalent static analysis for the models where 1st mode is the guiding one and no irregularities – plan, mass and stiffness irregularities are not present. The base shear can be computed by some factors multiplied by the seismic weight of the structure. The factors are specifically code specific.
If one do not get any specific code for equivalent static analysis, it is suggested that you go for time history analysis.