How to prepare effectively for teaching a graduate course?
I assume that you have something like a syllabus for the course, listing the topics that the students are expected to master. If you don't have even that then you need to obtain one, either from some prof who taught the course in the past, or by developing one yourself, perhaps by using a similar course syllabus as a guide. It needn't be terribly detailed at this stage, but needs a list of topics to be mastered. Secondary topics can be omitted, but noted. 

The next thing to focus on in the development of any course is what will be the student learning activities throughout the course. This is actually the most important thing. Choose projects, exercises, exams, or whatever, that will guarantee mastery of the key topics if done well. Lay the timeline of the course by laying out the sequence and duration of each of these activities. Make these things activities not just readings. 

With that done you can start to think about what you need to do to make it possible for the students to succeed in their activities. This might be lecture or it might be classroom activities (labs), etc. As you do this you can fill in secondary topics, realizing that you may have to forgo some of them if the students need more on the main ideas, but you will at least have some extra material to insert as needed, depending on how the course progresses. 

Find a way to get some anonymous feedback from your students along the way. One easy way is to invite students to submit (anonymous) questions and comments to you on index cards at the beginning or end of each face to face session. These questions should help you determine if the pace you have set for the course is right or not. Be prepared to make adjustments based on what they write. Find a way for the questions to get answered, perhaps in class, and perhaps in some electronic medium, even a mailing list. 

Note that the teaching philosophy behind this answer is that it is much more important what the student does in a course than what the professor does. Likewise, the feedback from students is very useful to a new teacher. 



Note that if the course is being taught for the first time, as happens in some fields, then some parts of this are more work than otherwise. Syllabus preparation will likely take longer. But in such situations, it is especially important to keep quite a lot of flexibility in the schedule and in the grading/marking scheme so that students wind up with a good experience: hard but not impossible.