Though my English is not that good, I will try my best to explain it for you. 乌鸦嘴 is a nouns. You can say "你这个乌鸦嘴。" or "他是个乌鸦嘴。". 乌鸦嘴是个名词， 用来形容一个人对好事的预测从来不说准，对坏事的预测却一说就准。 for example: --你别看现在天气这么好，说不定下午就下雨了呢 (Indeed the weather is very good now, but maybe it will rain this afternoon. Who knows?) （然后真的，非常出乎意料地，下雨了）(And it began to rain, unexpectedly) --你这个乌鸦嘴。（都因为是你乱说话才导致真的下雨了） (Because you said it could rain and it is really raining now! It is your words that brought the bad luck.) --我觉得他们肯定赢！（I believe they will definitely win!!!） --呵呵，谁都知道你是个乌鸦嘴，你这么一说他们肯定会输了。 (Well, as everyone knows your prediction never came true in the past. So I think they will lose the game, according to you.) --虽然我很自信我能赢，但是也说不定会输呢。 (Though I am confident that I will be the winner, it is possible that I may lose.) --不要乌鸦嘴。（不要说不吉利的话） (Don't say that. It may bring you a bad luck and become true!) （Actually, "乌鸦嘴" is used as a verb here. But this kind of usage is informal.）

Well, 烏鴉嘴 means you say sth inappropriate or sth bad and eventually it really happens!

不要烏鴉嘴 may mean "Don't jinx it" in English. Both your examples look okay, but perhaps the second one is less used.