How to effectively teach topic of frequency domain?
In my personal opinion, it's best to study how the master teachers of the field have presented this information, and to the extent possible, copy their approaches and their style.

There are two teachers in the field who have more or less taught a generation of DSP engineers their craft, either by direct teaching, or by teaching teachers.  They are Alan V. Oppenheim and Richard G. Lyons.  In Oppenheim's case, the Signals and Systems courses that he taught are still available on MIT's web site for free viewing, and his Discrete-Time Signal Processing is still the bible for many engineers on the topic.  Lyons still teaches and writes from Besser Associates, and his "Understanding Digital Signal Processing" book is simple and lucid.

You can't go too far wrong by copying the masters.