Explain the use situation before describing your mechanism
Begin with who uses the product, what object it interacts with, and what happens before and after use. A designer cannot judge a proposed mechanism from an isolated sketch. “A holder for a workshop tool” omits the tool's weight, how it is removed, the mounting surface, and whether the holder must accept multiple models. Those missing details affect both effort and safety.
Describe the problem in observable terms. Say that the user needs to return the tool with one hand while the other hand carries a workpiece, if that is the intended situation. Then identify which parts of your concept are mandatory and which are simply your current idea. A requirement to accommodate an existing mounting pattern is different from a preference for a particular curved shape.
NASA's logical-decomposition guidance distinguishes what a system must do from the design used to do it. For a small project, apply that principle with a three-column list: required behavior, reason, and proposed verification. This prevents a contractor from faithfully drawing an untested solution to a poorly stated problem.
Specify boundaries that drive the quote
List the mating objects and provide dimensions or physical samples. Identify the maximum overall envelope, desired adjustment range, approximate loads, and expected environment, marking uncertain values as assumptions requiring confirmation. Do not copy a dimension from an online photograph when a fit decision depends on it. State who will measure or supply the reference item.
Describe prototype fidelity explicitly. Is the deliverable a nonfunctional appearance model, a working bench assembly, or a sample made using a candidate production process? Include the expected number of units and whether units may differ intentionally. If the work includes electronics, software, packaging, or a separate mounting accessory, name those items individually rather than hiding them under “complete prototype.”
Set the handoff format: native editable CAD, neutral geometry, drawings for critical interfaces, purchased-part identifiers, and an assembly description. File access and design ownership need written agreement appropriate to your engagement. Technical handoff requirements are not substitutes for that agreement, but failing to specify them can leave you with a display model that another supplier cannot use.
A hypothetical brief for an adjustable book stand
Imagine a desktop book stand with three tilt positions and a removable page retainer. The first prototype is intended to test page access and adjustment, not shipping durability. The inventor supplies two reference books and defines a working area of 300 by 250 millimeters. Those numbers are scenario assumptions, not recommended dimensions for all stands.
The brief states that the stand must remain stable while a page is turned, and the tilt selector must operate without tools. It asks the designer to propose a measurable stability demonstration using the supplied books. It labels surface texture and final color as deferred. It also states that the prototype may use standard fasteners and visible supports if those do not obstruct the page-turning task.
Designer A quotes an attractive printed shell with no functional selector. Designer B quotes a rough working assembly plus a test video and CAD files. Their totals cannot be compared as competing prices for identical work. Return the scope table and ask each to mark included, excluded, or assumed for every deliverable. The process often reveals differences more important than the initial price.
Add one defined review after the first layout and one after assembly. A revision that changes tilt angles within the agreed arrangement differs from a request to fold the entire product flat. The brief should identify how substantial concept changes are estimated before work proceeds.
Make acceptance reviewable without pretending everything is known
A useful acceptance statement describes an observable deliverable under defined conditions. “Works smoothly” is vague. “The selector can move through all three positions with the supplied book in place, without an added tool or disassembly” is inspectable, although force and durability may still require later requirements. Explain which properties are intentionally not accepted at this stage.
Ask the contractor to return a list of unresolved assumptions before beginning. Review the consequential ones first: actual mating dimensions, material availability, whether the proposed geometry is manufacturable, and what test equipment is available. Do not let an assumption become an invisible requirement simply because it appeared in an email early in the project.
For changed requirements, retain the prior version, describe the change, and note its effect on cost, schedule, and previously completed tests. A numbered brief and dated drawing package are usually enough for an early project. Elaborate software is less important than ensuring everyone is using the same current version.
The brief you should be able to hand over
Finish with a compact document containing purpose, user scenario, reference objects, required functions, dimensional boundaries, prototype fidelity, exclusions, deliverables, review points, and open questions. Attach photographs and sketches with labels rather than embedding unexplained images. Use consistent units throughout.
Before sending it, ask a person unfamiliar with your idea to explain what will be delivered. If they describe a production-ready product when you intended a motion study, revise the wording. If they cannot identify how completion will be judged, improve the acceptance section.
The best brief gives a designer enough freedom to solve the problem and enough boundaries to price the work. Its value appears when you receive comparable proposals, understand the remaining assumptions, and can decide whether a completed prototype actually fulfills the agreed experiment.