You'll need a design file. STL format is preferred. OBJ and a few other types are supported, but STL is the primary one. If you don't have a design file, you'll need to make one using whatever tool you want. That is beyond the scope of this document.
Open the Slicer software. We prefer Cura, and it should be installed on many of the computers floating around the space. If it's not, install it! If you're using some other software, the steps may vary a bit from here out.
Drop your file into Cura. You'll want to make some adjustments to the file to make it print better (minimize need for support, etc.)
Make any changes necessary for the current print setup (speeds, temperature for the filament you're using, support material on/off, etc.)
Note the estimated print time. Cura does tend to be optimistic about print times.
When you get things the way you want, export the Gcode file to the SD card.
Eject the SD card and insert it into the printer (on the i3, it should go in upside down)
Turn the printer on if it's off, and go to the “Print” menu. Select your filename (sometimes they look a little garbled on the i3's display).
Wait for it to finish. (this step is a little vague and about a dozen things can go haywire at this point. Be aware!)
3D printers can generally be run unattended, but you should be at least peripherally aware of what it's doing if you're around. Filament can tangle- especially if it's been taken off and put back on the printer a number of times.