Besides a few common phrases to express disappointment, here comes a great video with a lot of examples used in different contexts to teach you how to use “Damp Squib”
See how this idiom is used:
The magic show last night turned out to be a damp squib.
It was a damp squib.
But then the rain came and turned the party into a damp squib.
The whole thing turned out to be a damp squib.
It turned out to be a damp squib this year.
Is there a similar expression or idiom in your language?
Of course, the best way to memorize this phrase is to keep using it over and over again. If you don’t have many friends who speak English as their mother tongue, do join one of the friendly communities of language learners 🙂 Best of luck!