Hi
The first thing to do is look well beyond any kind of technical design work. If you have an engineering background, either professionally or as a keen hobbyist, you'll be relatively well prepared to deal with whatever problems the design of the product throws your way, and these won't be the things you'll lose sleep over.
Think instead about:
- who is your target market, and how will you drum up interest amongst those people?
- what price could it sell for? Start with this figure, then work backwards to see if it can be made profitably at this selling price, given the costs of production, testing, packaging, delivery, VAT and reseller margins (plus, of course, your fixed costs that do not scale with volume).
- where will you have it made? What kind of setup charges can you expect, and will batch sizes be economic?
- who will sell it? Your own web site? A distributor? Retailers? Ebay? What will each of these options cost you?
- are you able to deal with warranty, support and returns?
- does your product require regulatory approval? Do you know what standards apply, and how and where to get it tested?
- professional indemnity and product liability insurance?