Once you have your STL, you must prepare it in a slicer program to turn it into instructions that the 3D printer can understand. Almost all of the self-service printers in the Innovation Lab are Prusa Mk4 machines, which use a program called PrusaSlicer. This page will walk you through how to prepare a print in PrusaSlicer, then briefly discuss the exceptions for other printers at the end.
There are a couple of ways to get files into PrusaSlicer
You can put multiple items onto the build plate at once. By default they will all be overlapping with each other at the center of the build plate, but you can click each on individually to move them out of each other's way.