For shelf prep, the key is this: practice questions.
They look like Step 1 questions, but they are NOT the same. They will require a certain measure of clinical thinking that you have probably not developed yet, but is the very thing you're supposed to learn during 3rd year. You know how in step 1 questions you tended to focus in on certain things and ignore the others? Well, you'll find yourself having to switch that up a bit as you focus on other parts of the stem.
Make this your goal: get through at least TWO complete sources of practice questions. I liked PreTest and Lange Q&A, but anything will do. Just make sure you get through ALL of BOTH. This gives you ~1200 questions and should cover pretty much everything on the shelf. Add in other resources as needed and do whatever mandatory didactics your school has, but make time to get through all your questions.
As for the rotation itself, have a good attitude and go in planning to work hard and see as much as you can see. It's a great time to build up your interviewing and physical exam skills, and your preceptor should allow you to see as many patients as possible. Depending on your future specialty, this may be one of the only times in your medical career where you get to be on the true front lines of the healthcare system seeing patients who haven't seen anyone else about their complaint. This is an interesting population that you don't often see in tertiary academic centers where everyone has been referred by someone else. It's an easy rotation to fall into the shadowing rut, but force yourself to see patients on your own and then read about their conditions. In a busy FM practice, you'll probably be able to see a substantial chunk of what's on your shelf in actual patients, something I personally find much easier to remember than what I only read in a book or question explanation.
So have fun! Look for the good parts in every rotation, even this one. Every rotation has it's elements of suckage and you'll find no shortage of fellow students willing to wax poetic about the suck on their own rotations. Just accept the suck and focus on the good parts 🙂