If I give a customer a plant list or a drawing, I still tell them that items are subject to change. One trip to the nursery to pick up what you think you want, changes in a second when you see the new arrivals, or a different size of this or that... I always drive thru my 3 or 4 favorite nurseries to check stock and availability. My best design work can happen there.
The process- for me- can be very on the fly and evolving. And the customer should know that- most every time they are cool as long as you stick within that estimate price somewhere. I try to place large items first, create general flows, and work down to smaller, filler items. What needs to be evergreen? Single item specimen here or mass there? Dedicated annual beds? What are my "go to" plants? What will the winter interest be? Flows and step-downs, how many different plan types to use? I always ask all these questions and more. It's hard work! And I've put plants in bad places....you just learn and go on.