What are the most important techniques for creating solid models for CNC machining?

Disagree, the first step is really understanding how the model will actually be machined. The best software in the world is not a replacement for real world experience. An understanding of ratio, depth, perspective, outline and materials used are much more important than a software that can make it appear good on a screen. That being said, without good software, 3D modeling for CAM would not really be possible. CAD/CAM can create 3d cabinets out of 2D parts, but if you dint have an understanding of cabinet making, the software will do you little good. Know your outcome, know the process, then proceed to design.

Three things I find very helpful are: 1) Know how you plan to manufacture a part prior to designing when possible rather than the all too typical approach of making a complex part then trying to find a manufacturing method that can accommodate the rigid manufacturing methods. 2) Model your part in a way that simulates the operations that will be used to manufacture a part. This is a trick I learned from an internship a while ago and have not heard since. Sometimes taking one or two extra steps in a design tree you may save a few operations in manufacturing and simplify the process. A few extra minutes in the design phase can save manufacturing costs over the life of the product.

I agree with Grant, after you know what you want to create, and more importantly, HOW it will actually be created, you can move onto software choices and design. Some simple 3D (2.5D unless you are using min 4 axis) designs can be created with simple software. Complex outcomes more often need more complex software. I'd suggest some tutorials, trial versions and plenty of reading, then, find a software that suits the majority of your needs, matches your skill level and is capable of outcomes you desire. Most design skills are essentially transferable to a point, between software, so find something that works for where you are and what you know how to create, learn and grow from there.

I'd agree that the 3rd step is to get into actual design. Unless of course you are seasoned and experienced, in which case you understand steps 1; understanding how the model will actually be machined, 3D print, 3/4+ axis spindle, etc (all of which is cnc) 2; choose the appropriate software .. in that case steo 3 is actually step 1.. but that only comes in time. Regardless, once you get to design, you will already have a great understanding of your machines capability, so your design can match and align with your actual capabilities. Obviously, you can't machine a 5'x10' model on a 2'x2' bed, not in one piece anyway. Design considerations, beyond machine capabilities, I think, are a whole nother subject.

After preliminary design you should finalize your tool selection and toolpathing strategy. Be prepared to refine your design if necessary, after these steps. Specific tools should have definitely been considered in the design process, but tooling doesn't have to define the design. Multiple tools can be used to enhance detail or accentuate and change aspects. With a good design in place, it's time to narrow in on the final machining process; what tools are required and what are the actual toolpaths/toolpath strategy you will use for each detail and process. As with most things in life, there are several ways to achieve desired results. Now is the time to test and simulate the results of your various tools and toolpath strategies.

If it were always as simple as 5 steps, then I wouldn't have to try to combine multiple steps into 1 lol So, assuming you understand the output machines capabilities, have a grasp and understanding of your chosen software, know and understand your tooling choices and tooling strategies and have created a design that at least works in simulation.. then you should do some test cuts. Prepare the machine, tools and material, then isolate a section of the model and run a sample. If results are not as expected, then refine as required. If results are as expected, then test the model in its entirety, ensure your design and model is complete and reacts as expected and now you have a model that is solid for CAM (CNC) machining.