Deliberate Failure
Jono Dry’s How I Plan My Drawings has a nice suggestion for how to zero in on the boundaries of an idea (in the context of an artistic work, but it seems likely more widely applicable).
Namely — intentionally increase the failure rate of what you’re trying to accomplish along deliberate lines to “cut off the excess around a […] vague idea that you have”.
(Other more concrete insights into his artistic process there as well.)