How can I go from rough sketch to prototype design without getting burned lol

Got burned on my last prototype attempt. Paid a deposit to a firm that promised the world, received something that I didn't work and a bill that kept growing. Lost close to 18k before I pulled the plug.

Now I have got a new product idea I want to develop and I am trying not to repeat that. Bigger budget, a lot more careful this time.

The part I got wrong before was the vetting. I picked based on the website and a sales call with someone who seemed confident. Never dug into the actual technical side of what they'd be doing or asked the questions that would have told me whether they actually knew the work.

What vetting signals actually matter before you commit to a prototype design firm?