I’ve seen the usual advice on protecting your idea if you hope to patent it. Inventors are told to require anybody with whom they share the idea to sign a nondisclosure agreement first. They know they will need to do a patent search to learn if the idea has already been patented. They receive recommendations to build a prototype or take some other concrete action toward ownership of the idea.
Start a diary/journal in a bound notebook. Include the date of the invention (when the idea was initially conceived) and a description. This is important in connection with filing for a patent in the future as the United States is a ”First-to-Invent” country. Keep the journal updated as you develop prototypes, make modifications, etc.
Does it have to be a bound notebook? Maybe, maybe not. That’s easier to protect than an electronic journal.
Another idea might be to write a letter describing the idea and mail it to yourself. Save it in a safe place without opening it. The idea is to have a provable date that shows you invented the idea first, and you’ll have a postmark on the letter.
You can find other information for inventors at United Inventors Association.