You'll frequently come across difficulties of creating system/procedure and getting control over operations, if you're a design service provider/ about to make new product for your company. Well, here are few guidelines which I've learnt throughout my experience in design and manufacturing service industry.
The product development procedure starts from prototype making and has never end/ECO or can be said ends at production.
Prototype: Demo samples are made to verify the design and mainly functional tests done on these samples.
Pre-launch: 100-1000 product samples are been sold in market to check market response of new developed product.
Production: This is where mass production starts and product process validation done.
ECO: Up-gradation of product through engineering change management.
Let's see for prototype. Divide overall project development into below phases.
Concept - It is a idea understanding phase. Search for already available data, images, diagrams, technical details etc over internet, books or learn from experts. Prepare a product block diagram and process flow chart. Also make a overall summary report.
Design - This is where you actually starts designing your product. Make test plan, algorithms, programming on code, circuit diagram, PCB design, material selection, hardware specs, design BOM, architecture etc.
Plan - It is a pre-checking/readiness phase before actually going for assembly and box build. Order material as per BOM and test it visually/ electrically/ mechanically whichever is applicable. Make assembly procedure/ operations document like visual aids, work instructions, checklists, forms etc. Check if all required equipment are available and man power is well trained. Usually it is better if the R&D engineer himself do the assembly for prototypes as he's well aware about the product design.
Prototype - Here you actually do assembly and check performance. Do all the tests as per test plans and track record. Make a post-mortem report on overall project on findings and make corrections before going for next samples. You can also do risk assessment by using some quality tools and FMEA