While there is no substitute for coming to a class and learning from one of the fine instructors we have, there are things you can get from books and the web in general. This is an non exhaustive list.
• Nariyama Shihan's book: Aikido Randori is a must have.
• Scott Allbright's book: Aikido and Randori: Reconciliation of Two Opposing Forces is another good book to have.
Full grading syllabus including the required mat hours:
How to tie your belt