Thankfully this concept is relatively straightforward.
The idea is you have a .STL file - whether it was downloaded online or designed by you in 3D-modeling software - and you want to bring it into a software environment that sets the .STL up for 3D-printing.
We do not refer to this as 'opening the .STL' because that would only view the .STL contours. Most 3D-printer software cannot actually 'edit' .STLs, so we therefore do not 'open' .STLs in 3D-printer software. Instead we import the file.
The process is simple. For example:
Launch the software required by the printer you're going to use.
(For MakerBot printers they use MakerBot Print; for Prusa printers they use PrusaSlicer; for Ultimakers we recommend Cura; etc. There are many different slicer programs - these are just a few of them.)
There is always a menu, and typically buttons that perform the same function as menu options.
The function you're looking for is the import file function - sometimes called "insert" or "add."
You want to import a .STL into the 3D-printer's virtual build plate.
For example with MakerBot Print you can either click File -> Insert File:
Or you can use the big buttons to do the same: you begin a new "Project" then "Add Models" to the build plate:
For the MakerBot Print software "Insert File" and "Add Models" are functionally the same thing - they're just called differently. This may be an oversight of the developer process or it may be intentional - we'll never know!
The process is similar-but-different for PrusaSlicer. The menu option is File -> Import -> Import STL/...:
To perform this function graphically you simply click the 'add' icon on the dock:
( you can also see there's a keyboard shortcut (for macOS): ⌘I )
The reason we don't call this process 'open' is two-fold.
We've previously explained that opening the .STL is really just viewing its contours.
The other reason is because if you're in a 3D-printer software environment, typically to open a file in that environment means you are opening a "project" or "file" associated with that environment. This project file is critical. It contains the imported .STL along with all the information associated with the setup process - the settings!
Imagine the software environment has 30 different settings. You import a .STL and then setup the print, changing 25 of the 30 default settings along the way. You print the object but you want to go back and change one setting. You don't want to have to change 25 settings again. Simply open the saved project, and change the one setting you want to adjust - the other 24 changed settings were automatically implemented when you opened the project.
Typically we use the word 'open' when we are referring to a "project" file or file associated with a specific 3D-printer software environment and we use the word 'import' when we are referring to inserting a .STL in a 3D-printer software environment.
Now that you've imported a .STL to the build plate, it's time to slice the file for printing!
