Mock ups can be as simple as some scribbles on a napkin or as complex as wire frame modeling accompanied by hand carved plastic model structures. In the end, it’s not the complexity of the mock up but how well and how fast it conveys the idea of the design.
My personal preference is a white board, some paper / pens / markers / pencils and a graphics program. If I’ve got sufficient information I can usually come up with a viable mock up that transfers into design, development and production pretty quickly.
There’s a great many ways to do mock ups so find the way that works best for you. Read whatever books you need to in order to learn different ways that you can adapt to your need; but, makes sure you’re doing it your way because your way is going to allow you to explain it, if need be, and create a much smaller transition time from mock up to design.