What is an STL?
An STL file is a representation of a 3D object (in this case, your design) in a format your 3D printing software can understand.
What do I do with it?
The first step is to open the STL file in your 3D printer’s software. Each printer uses a different software program - consult your printer's manual if you aren't sure on which you have.
Once you’ve got the STL open in your 3D printer software, you can position and size it as you see fit.
If you have multiple STLs you want to print you can add them, too. Just remember, the more models you add or the larger you scale them the longer the print is going to take.