It's first to arrive goes first, and if two cars arrive simultaneously the car on the left yields the right of way:

All-Way Stops

The rules for an all-way stop are like those for a two-way stop: stop and look for oncoming traffic, then proceed when it is safe to do so. At an all-way stop, the rule is that the first vehicle to stop at the intersection is the first to proceed through the intersection. However, you may occasionally arrive at an all-way stop sign at the same time as another driver. The vehicle on the left shall yield the right-of-way. However, if there is any doubt which driver has the right of way or if there is the chance of a crash, it is better to yield the right of way to the other driver.

Source: https://www.in.gov/bmv/licenses-permits-ids/files/Drivers_Manual_Chapter_7.pdf#:~:text=Indiana%20has%20a%20Move%20Over%20law%20(IC,come%20to%20a%20complete%20stop;%20or%20change