If the set up will be temporary I would tie the generator into the Service Disconnecting means load side wiring if possible. This presumes that the generator has a main breaker that will serve in place of the Service Disconnecting Means. It also presumes that the service disconnecting means is a separate piece of equipment that is connected to the load by field installed wiring.
If, on the other hand, the service equipment is some sort of combined main breaker and panel equipment then you will need to use a double block and break technique to isolate the building from the utility power. If the building is supplied through a 400 ampere meter socket than you would open the service disconnect, pull the meter, blank off the meter socket and disconnect the meter load wiring from the service disconnecting means. You would then blind off the meter load wiring with insulating materials. That makes the line side terminals of the service disconnecting means available for connecting the generator to. Since meter sockets in the four hundred ampere size are often equipped with shunts you will have to secure the shunt in the open position. The same lever is often used to release the meter jaws and engage the shunt bypass. These have to be operated to the open position after having removed the meter. If the service is larger than four hundred amperes then removing the meter will not deenergize the service disconnecting means line side terminals. So check for voltage every step of the way.