STL (stereolithography) files are commonly used for 3D printing and are widely available for download from various sources. However, opening an STL file in Blender can be tricky if you are not familiar with the process. Here's a step-by-step guide to help you open and work with STL files in Blender.

1. Download and Install Required Add-ons: Blender does not natively support STL files, so you will need to download and install a free add-on called 'Import-Export: STL format' from the Blender Add-ons section. Once installed, make sure to enable the add-on in the User Preferences window.

2. Open Blender and Go to File > User Preferences: In the User Preferences window, click on the Add-ons tab and search for 'STL'. Check the box next to 'Import-Export: STL format' to enable the add-on. Close the User Preferences window to save the changes.

3. Import the STL File: With the add-on enabled, go to File > Import > STL (.stl) and navigate to the location of your STL file. Select the file and click 'Import STL' to import it into the Blender workspace.

4. Adjust Scale and Orientation: Depending on the source of the STL file, you may need to adjust the scale and orientation of the imported object. Use the Scale and Rotate tools in Blender to make necessary adjustments.

5. Convert to Editable Mesh: By default, imported STL files are treated as a mesh object in Blender. If you need to make further edits or modifications, you can convert the mesh to an editable object by applying any necessary modifiers or using the Remesh or Retopology tools.

6. Save and Export: Once you have finished working with the STL file in Blender, you can save your project and export the modified file in STL format by going to File > Export > STL (.stl).

By following these steps, you can successfully open, work with, and export STL files in Blender for 3D modeling, rendering, and 3D printing. Remember to always credit the original source of the STL file if it is not your own creation. Happy modeling!