Step by step.
Log in or sign up to www.tinkercad.com
Choose “New Design”
Choose your .STL file from your PC library.
When the item has been imported selected with the mouse with a rectangle surrounding the whole object.
Move the object to left or right of the build area.
Click the “copy” icon (top left, two squares).
Click the “paste” icon (next to copy, a clipboard and a square).
Drag the pasted item clear of the original.
Click the “Mirror” icon ( top right centre, 2 triangles and a vertical line).
Three double-headed arrows should appear. Select the direction you want to reflect the item ( normally left-right).
You should now have two mirror-image items.
Drag them together if you need to.
If you want to save this as a single item, select everything with a rectangle as before and click the “group” icon ( a circle and arrow-head combined).
Then select “Export”
Choose “Everything in the design, and .stl
Then Save As (up arrow from Save choice)
I hope this has been helpful.