FF: No, you don't need a published IP, or to call "initials" - it's actually very easy and safe.
Just fly parallel to the runway while descending for the last couple of miles (or from the edge of the ATZ), call "joining deadside", fly along the deadside of the runway at circuit height (or very slightly below so you can see downwind traffic against the sky), and then turn onto downwind at an appropriate point to fit-in.
IMNSHO straight-in approaches at uncontrolled fields should be banned
I'm somewhat surprised that you weren't taught OHJs, but I guess it just got missed by accident.
HFD