Decide on a product. Different products lay different ways. So this is important. You make need a general measure prior to deciding on the material so you will know what your quantity requirements are.
Decide on an installer. Look for references. Make sure they have experience installing your product.
Let the installer measure. If they have experience with your product, they will know where to place seams, what direction to lay the flooring, what works well on stairs, how the flooring needs to lay in halls and in front of windows and doors, how to minimize material and waste, making sure to include material for closets, and what overage allowance to add in. They may be able to present some cost-saving ideas such as using more seams to minimize waste.
They will also be able to coordinate the delivery and installation schedule.
If the installer does not have experience with your chosen flooring product, or cannot provide you with reliable references, you may experience expensive installation surprises and problems.
If you overbuy material from a bad measure, you will have to either keep the leftover material or pay a restocking fee to return it. These are both expenses that can be prevented by getting a good measure.