This is one of the most important points in German, and the German language is (in)famous for its endings, so you've likely heard of them already. Here you want to be sure to take small steps and you want to avoid trying to learn all the endings in all the cases all at once.
It bears repeating to be sure to learn these adjective endings one case at a time.
Review the transcript, study the lesson and do the quizzes: www.newsinslowgerman.com/series/guts/24/transcript