The workflow for me goes: design software -> output design to .stl file -> import .stl into slicing software and set up the layers / temperatures / settings as desired -> export g.code to SD card -> plug SD card into printer and run g.code. I've been using Cura for slicing software, but there are other options as well. The slicing software breaks your model down into many layers (or slices, hence the name) and tells when and where to move the extruder to make each layer.