Start with the operation
Describe the operation before describing the object. Name what the operator receives, what must happen, and what acceptable completion looks like. Record the current failure: a wandering drill, a reversed connector, inconsistent masking, slow inspection or an awkward camera adjustment. Include the part quantity, shift pattern and expected fixture life. This separates a useful production aid from a model that merely fits one sample.
Name the datum
A fixture must locate from stable, intentional surfaces. Identify the primary face, secondary edge and any tertiary feature used to remove the final degree of freedom. Provide written dimensions and tolerances from those datums; photographs and scale references help orientation but do not control manufacture. If parts vary, share the expected range and several representative samples rather than a single ideal component.
Close the approval loop
Agree which dimensions are critical, which surfaces may be touched and which process loads matter. The proof should carry a revision ID and a plain-language list of open assumptions. One prototype round checks seating, access and ergonomics; it does not certify the customer’s machine or process. Production begins only after written approval, and later changes become a new revision so spares remain traceable.