go off half-cocked
(figurative) to go into action too early or without thinking. (Originally refers to a flintlock or matchlock gun firing prematurely, before the trigger was pulled.)
Don't go off half-cocked. Plan out what you're going to do.
Bill went off half-cocked and told everybody he was running for the state legislature.
This website says We now commonly use 'go off at half-cock' or, in America, 'go off half-cocked', to mean 'speak or act impulsively and without proper preparation'. But I think that's probably written by an American who mistakenly supposes anything strange-sounding or archaic must be British. As this NGram shows, go off at half-cock never had any significant currency.
You'll also notice from that NGram that go off half-cock (no at, no -ed) is far from unknown. This is because in most spoken contexts it would be almost impossible for native speakers to hear that -ed anyway, so if they're not familiar with the origin they wouldn't necessarily realise it's a "past tense verb" being used adjectivally. But competent native speakers wouldn't usually get that "wrong".