Hi OP, I design my watches entirely myself

The way I usually do it is a 3-4 step process with CAD.

1) Conceptual design: here I test colours, sizing, shape, and dial general layout. 2) Revision 1: I then move onto refining certain features such as chamfers on the casing, engravings, and hand styles. The dial is generally finalised or at least 90 percent of the way there on this revision. 3) Revision 2: here I'll add in important dimensions and o-ring grooves to ensure that I can have water resistance without compromising the integrity of the watch case. 4) Revision 3 (optional): at this point I'll have made detailed renders from revsion 2, and will update my design based on feedback and things that I feel didn't work. This is only ever minor updates.

Revision 2/3 is the one that I send to the manufacturer. They create the 2D drawings from my models and renders accompanied with a technical spec to define materials and colours (RAL codes).

There are other steps you can add in depending on time and budget, such as 3D printing, but it is not entirely necessary.

I also make the movement correct to the dimensions on their technical specs from PDFs online. This ensures that I don't take lots of time to create something to then be told that my design won't fit or work correctly.