My teacher's teaching style isn't working for me, and he asked for feedback about the class and how he could teach it better. He teaches with powerpoint and gives quizzes that are all about rote memorization of the slides. Most of the class finds these quizzes to be of little help in understanding the information, but nobody knows what to say to the teacher about it.
Basically, I agree that what xLeitix said is the gist of if: as long as you are respectful, you should be fine.
To elaborate a little, my only experience with giving feedback back to the professor is a bit specific: a fairly young professor, that just got the course for the first time and was trying to improve it. Like in your case, he also said that he would appreciate feedback.
I'll try to give some specific advice, with a specific example of my situation:
again, be very respectful
(In my situation, this meant that I didn't start loudly complaining in class as the reading material quality (made by the prof) steadily degraded during the year. Rather, when I found enough time, I passed by the professors office during the office hours and asked him if he would be interested in my feedback on his reading materials.)
be as informed as you can about the coursework for which you disagree with the teaching approach
(Basically, why would I take your advice on how to teach X if you don't even have a rudimentary understanding of X?)
be specific, and give argumented reasons
(For me, this meant, when he agreed to listen to my feedback -- he actually scheduled a meeting outside of his office hours for that class suspecting it will take some time -- I didn't just come and said the materials were getting worse. I had specific examples of pages and exercises that I had trouble solving, marked in the script. I also offered information like "it took me X hours to finish and understand the first script (good quality), while it took me 4*X hours to finish the second one (worse quality), and they are supposed to cover approximately the same amount of coursework". For you, it might mean explaining how and why you don't think the feedback you're getting from quizzes is not helpful.)
make specific recommendations or suggestion (but don't demand anything)
(Again, in my case, alongside some of the things I marked, like exercises and definitions I had trouble with, sometimes I had my own versions of definitions, or I would re-phrase the examples in a way that made it easier for me to understand. It also help pointing out the ambiguities in the original and saying why you think your formulation resolves it.)
be committed. The bigger the change you want to make, the more committed your will need to be.
(As you can see form the above, making a substantial change, at least according to me, will probably require you do to a substantial amount of work on your own -- without the professor's help -- to be able to give suggestions well supported by arguments. Of course, you don't have to take all the points super-seriously if you just opt for a short 5-minute chat giving some suggestions and examples.
On the other hand, in my case, where I loved the course, I thought it was important, I thought the prof was great and trying really hard, and I had a (young and naive) wish to make a change for future generations, I took a few days to prepare my notes, I came to Uni a few days after the summer break has started in order to have time to give all my comments in as much detail as the professor wanted, and it did help that the prof actually knew and remembered me from the courses and valued my opinion at least a little.)
If the quizzes are of little help to everyone, and the teacher is asking for feedback on his teaching style, and nobody says anything, then the blame is on the students. If the teacher is sincerely asking for feedback he will not be offended by your comments (given you express them in a polite way). Tell him politely (and directly) what the problem is. He will surely appreciate the feedback.
As an example, I am teaching programming to some students with little programming experience. They are struggling and I know some of the concepts are difficult to understand. At the end of every two classes I ask if the explanations and exercises are being helpful, if they are helping them to understand the topic. I always get positive feedback. That makes me a little nervous- if they are failing to communicate their difficulties I will fail to make them understand the topic. I would prefer constructive criticism than misunderstood politeness.
If it were me, I would leave an anonymous typed note that cannot trace back to me. No one responds to emotional feedback (that only makes a person defensive and even more resistant to improvement), but make sure the feedback is highly constructive, and entirely unemotional and not accusatory in any fashion.
When I was in school, I did confront bad teachers. I can confirm that direct, accusatory methods do not work. Now that I am older, I can say with certainty that the most unemotional and constructive methods will work best.
If you have to, first write the note when you are emotional, and then come back some time later when you are no longer emotional and rewrite the note, and then, only then, give it to the teacher. Try to give it to them anonymously, because if it's really a bad teacher, they might single you out, and that's the last thing you want.
Alternatively, if you have built a strong relationship with the teacher, then you can use your relationship to try to help them and give them valuable feedback, as a friend. People usually listen to their friends who speak out of a kind heart.