Citation in lecture notes?
There are two questions here: 


how to cite?
can (and how) such materials be used?


The first question can be certainly answered. Library Guides from Victoria University has the following example in APA style:


  Ooi, D. (2018). AEB1804: Young People in a Global Community, week 1, session 1 notes [Course Presentation]. Retrieved from https://vucollaborate.vu.edu.au/d2l/le/content/177492/viewContent/2217850/View


This question also deals with citing lecture notes and when to do it. Concrete example for lab manuals and lecture notes.

An important discussion on whether to use citations while preparing your lecture notes can give a perspective on why doing proper citations in lecture notes is a good idea.

However, citing lecture notes has its limitations, since lecture notes are essentially gray literature.

Now, to the second question whether and how you can use them, which is a question of copyright. This question discusses the copyright-issues-free preparation of lecture notes to get you started. 

Lecture notes rarely mention a license under which they are published, so you cannot assume they are in the public domain or under a permissive license. You can contact the author of the notes (or the copyright holder, which might be university). This will raise two problems (that theoretically can be resolved):


sometimes it is actually unclear who is a copyright holder for the lecture notes
you rely on the fact that the lecture notes you are trying to use do not infringe copyrights on their own




So, I would suggest the following. Use the material from the lecture notes in a way that will constitute fair use. Citing paragraphs here and there would certainly be fair use while using images and figures might not be, as well as using entire chapters. Use appropriate citations where applicable. Also, consider asking the author of the notes for an explicit permission.

Known assumptions and limitations of this answer:


you do not intend to sell your lecture notes
copyright law is complicated and has its own peculiarities in every country
you intend a fair use (in a common, not legal, sense) and wish to be as fair and copyright-respectful as possible