Some students of Aikido keep a journal of what they learn in class. Taking notes on the mat is impractical at best, so if you are going to take notes, it will have to be done after class. This is best done immediately after class when things are still fresh in your mind. If you can't recall the technique names, ask one of your fellow students. They are usually glad to help.

If you are artistically inclined, try drawing diagrams of the important points or concepts.

(insert a diagram here)

Much of Aikido just has to be practiced over and over until your body remembers what to do. Journaling (note taking) can help you to learn the philosophy, conceptual points, dangers, short cuts and tricks, etc.