A very simple introduction to 入聲 is that they are syllables that end in -p, -t or -k in Cantonese.
Typically, they end in -e in Mandarin.
Possibly because the tone mapping between Cantonese and Mandarin syllables for 入聲 syllables is quite irregular, I find it difficult to remember what the Mandarin pronunciation is for such syllables.
But then, so do my friends from mainland China who are learning Cantonese.
It took 50 repetitions before I could remember that 學/hok6 was pronounced xue2 in Mandarin.
It took my mainland friend 50 repetitions before he could remember that 學/xue2 was pronounced hok6 in Cantonese.
By contrast, it took 5 repetitions to remember that 港/gong2 was pronounced gang3 in Mandarin.
My mainland friend needed 5 repetitions to remember the same thing in reverse.
Difficult 入聲 syllables: 十 一 七 吸 及 甩
Easy non-入聲 syllables: 我 香 台 電 中 氣
Certain aspects of learning a second Chinese language are difficult for people who already speak one, (so don’t be discouraged by interference effects when learning L3.)
That is all.
(See assumptions used in this post here.)