You can take your rental car on the ferries provided the car rental agency allows it. You should check with them (if it's not clearly stated, read the Terms & Conditions and/or contact them). Some charge an additional fee.
To get to Kefalonia, you'll need to drive to Patras and take the daily Levante Ferries ferry (3.5 hours crossing) which arrives at the port of Sami.
Timetables and reservations are not available very far in advance (probably no more than 2 weeks). You'll need to check the Levante Ferries website.
https://www.levanteferries.com/en/
Prices are available here:
https://www.levanteferries.com/en/times/
If you don't know the exact date of your trip to Kefalonia, it's not essential to book, although it's generally recommended if you have a car. If you book online, you will be asked for your car's license plate number. If you don't know it, which is often the case with a rental car, simply enter RENT123 (or something similar).
Your rental car will be in the <4m25 category, (except if you rent a van or a very big car
You'll get e-tickets that you will need to present upon boarding.
With a car, plan to be there an hour before the ferry departure. Only the driver of the vehicle is allowed in the car at the time of boarding, other passengers must board separately.
See an example of how to board the ferry with the car in this video:
https://youtu.be/0BU7fOu_IfM
From Kefalonia, there are options to go to Ithaca, but not necessarily from the port of Sami (there are 3 ports in Kefalonia). Check with the ferry ticket agencies when you'll bein Kefalonia.
Be careful of weather conditions in November. Don't get stuck on an island because bad weather prevents the ferries from sailing. Weather forecast for Kefalonia:
https://meteo.gr/cf-en.cfm?city_id=9
Wind forecasts above 7 Bf may mean no ferries.