What's the best way to start the interior design process?

I think you slould start with studing the function of the area and how to make it so comfortable to the users and to make them happy .
And by simulating how the people will using it in the real world you reach a good design

well you need freehand sketches to be able to give your concepts and design thoughts form away from the limitations that a 3D software could impose on you in the "design brain storming " process....
however, it's common that your client wont be able to understand/ visualize your sketches (even if they are perspective sketches) so once you've finalized what you want, you can turn to the 3D software and do a rough 3D modeling of it (nothing too fancy or that acurate) and show it to the client to see if you are on the right track.....once the client approves the concept you can move to making elaborate rendered 3D until the client is fully happy with what they see :)

Best way to start is to do brain storming with your client, so you can find out his/her decorating style, and discuss about the main elements of your design..
example: Space, Color, Texture, Form...
etc.
This first step won't necessary give the interior a 100% clear image of the final design, but it will be his guiding key for the suitable direction of the his way of thinking..

I think 3D render can't match Man's free hand Sense of creating very fine , clear , fast developing Design before Rushing to Rendering while on the other side starting with 3d max colorful rendered images will push you to reach a fast unsatisfying low conceptual Designs and Developing after then would be much harder , and you'll have to go back to sketching to start developing so Starting with Free hand Sketches would be faster and saves time

My Opinion is to start making an idea first ( Concept ) after collecting the available information.

Freehand sketches for sure because its your guide line beside this way you will make sure you clearly understood your client thoughts ideas he will be able to tell you what and what not to do through the meeting before you start the modeling process.

The three best way to start the interior design process for me are;1.
collect all information regarding the rules and regulations from the building administrator, wherein the proposed project will be done.2.
collect all requirements by the client and study the possibility- if the proposed design is fitted or beneficial for the client me as a designer and for my company as well.3.
study the existing government laws- to apply green architecture, feasibility with regards to the public and environment.
after wards.
if possible in front of your client upon collecting the requirements from him/she, try to draw freehand sketches by way of striking your pen, in a coupon bond size, most probably interior worm view- (as per my exercise since then).
as clearly as you can, that your client can visualized immediately of what his point of view.
Sure your client will be more interested to get into the project be done ASAP...
he he..
For me that was effective and easy once i get started in creating3d's max perspective, then putting in Primavera P6 T- schedule as draft-1, prior for presenting to the client and recieve the initial down payment .
he he he .

The best way is to consider client brief together with the actual space available interm of circulation and choice of materials.

start by Mood Boards presentation

i think its a gradual process, and all you do must be to the client's satisfaction1-briefing from the client to know his requirements( which can be an office, commercial, or residential client.) in the case of office, know the organisational structure and always plan for future positions to made in the office space.2-the clients budget to ensure cost minimization3-consider the space you wish to use and knowing the measurements and size of the space4-think of what will fit into the space produced5-come up with a sketch to understand the ratio of movements by humans to the ratio of furniture that will be produced to make sure there are no interruptions of movements6-consider the emergency exits for quick access out of the building to ensure safety of everyone inside the space.7-3D Designing after all these, you can finally tender the proposed interior design for editing or changes.
i hope this tells you all you need to know about interior design process.

first study the needs after that let the imagination begin sketch down on paper built it in screen using Autocad and3ds max after you finish it off start taking some pictures make your presentation board and have fun