Seismic Shear Force applied in Center of gravity

Good Day Everyone

I do have a quick question regarding Seismic Load.

Q1. If I have the total mass for seismic design as Force (kN), then I apply it to center of mass/gravity of my structure, How can I transfer it to the nodes in the structure? Is it safe to use Master Slave? 

Q2. In relation to Q1, if ever Response Spectrum is recommended for seismic force calculation, I really don't have the experience to operate in STAAD.Pro. I would gladly ask for a procedure on how to operate.

By the way, I only have a small platform with only one elevation as the floro diaphragm.

Thank you in Advance.