Assalam-o-Alykum,
The letters with Qalqala, does it ring a bell in your mind. Yes it did for me but I had forgotten those words and also the meaning of "Qalqala".
Simply speaking "Qalqala" is like you throw a tennis ball on the floor and it bounces back to you. You say these letters and the sound should come back to you.
Below are the letters where you have to do Qalqala but only when there is a 'sakoon' or what we call in Urdu 'jazm' on it.
ق, ط, ب , ج, د
If you fear that you will forget these words after closing the blog, just memorize the word (Qutub Jadd) and this will cover the letters like قطب which means 'pole' in Arabic and جد which means 'with' in Arabic.
Now coming back how you have to perform the Qalqala task, look at the Surah Falaq:
Look at the last letter which is ق, and since the ayah is ending here, so you may like to stop here. When you stop, it becomes a qalqala letter. You will say ق with a sound that comes back to you. Falaqa. So you notice that I added an 'A' after ق, but you don't have to say complete a but rather half 'A' sound.
Practice it with other Ayahs of Surah Falaq as it provides one of the best examples for Huroof-ul-Qalqala.
Wish you best of all practicing these words and hope you will enjoy.