There are two ways to save an individual symbol - saving the entire file or dragging selected objects directly into the Library.

Saving a File as a Symbol

If you create a symbol as its own file, simply save the file in a folder already identified as a library folder. Or you can save it to any folder, then load that folder into the Library palette.
For example:

1. Create a new door and save the file as "FrenchDoor.tcw" in the "Symbols\Sample Symbols\3D Symbols\Door" folder.
2. Open a new file, open the Library Palette, and open the "Door" folder.
3. The "French Door" symbol appears in the palette, and you can drag it into the drawing.

Saving Selected Objects as a Symbol

To save objects as a symbol, select them and create a group.

(It is not essential to make a group, but the symbol will be easier to work with later.)
Open the Library palette, open a folder, and drag the objects by their reference point (the yellow circle) into the Palette. You will be prompted for a symbol name.