I am making some lecture notes for a Computer Science course in my mother language, which is Italian. Some of my notes are based mostly on lecture notes by other professors. I do not copy and paste, but I paraphrase and put my own opinions in portions of the lecture notes. Apart from the references, I usually put this quote at the beginning of my lecture notes when it is necessary:
"The following material is heavily based on the lecture notes by Professor X given at the University of Y during the course Z"
Is that enough? I would not like to be blamed for plagiarism or any academic misbehaviour for making a silly mistake.