Flowrate * density * head and divide by the pump's efficiency at that flowrate. Add IMO, about 20 to 25% and you'll get close to sufficient power required. Pick a motor size from the standard sizes available.
Examine performance from start-up speed to maximum operating speed.
Determine if you need additional controls.
Lastly, find some engineer that has experience with pump applications to review your work. Internet answers or supervision don't count. This part is the most important aspect of this problem.
If you have selected the pump from a pump performance curve it would normally have the power curve shown as well, if not, the easy way to calculate the power at the point of consideration is,
Q*H*SG/102*E = kW
Q = (litres/sec)
H= (metres)
E= pump efficiency - as a decimal - at the point of consideration
S.G. = 1 if water
102 = constant to assemble all the other factors and conversions.
This gives you the power requirement at the "duty point", from here you need to consider a number of other factors which comes down to experience of pumping systems;
can the pump run-out further on its performance curve at any time during operation?
is the pump duty constant or variable?
is S.G. constant?
what are the start-up charactristics of the system?
is voltage to the motor constant?
factor in ambient temperature and altitude and dusty operating conditions if applicable.
What is important is to select a motor that will not be forced to operate in an overload condition at any point within its application / hydraulic range - if this is not possible other measures must be introduced to protect the motor.
In most cases, the motor size is dictated by what is available from the standard range of motors up to around 200kW over this size you can usually specify the rating that you need.
Just a clarification for something that may or may not be obvious to all. There would be no point in selecting a motor power rating for a pump's maximum flow capacity, if the piping or process flow will not also reach that capacity. Superimpose the pump's curve on the system curve to find the maximum power required from the perspective of operating the system as a whole.
