At first, it was hard since I've never owned a model 3. Even if you are a model 3 owner, you are probably used to using your phone as the key.
They give you a keycard.
anyway, I didn't know how to lock the doors. finally I figured out the lock symbol on the central display and locked the doors. But when I got out, they were unlocked again. The trick was to open the door first, press the lock icon, get out and close the doors.
The easy way to lock the doors... Well, you can google it and find out, it is not intuitive, but easy once you learn.
Now renting a car that can drive itself was wonderful. I had to put a lot of miles on and it helped.
Finding charging is easy -- it is all built into the display. You can navigate and it will integrate charging into your trip. I was already familiar with this, but if you're unfamiliar it might be something to learn. There ARE superchargers everywhere, and L2 chargers everywhere else.
I had to do level 2 charging (J1772) and it I had to call hertz to figure out where the adapter was. (It was in a bag in the footwell of the trunk). Once I had that I could charge at hotels.
Hertz did spam me with lots of "get to know the model 3 before your trip" stuff. I should have read it a bit.