The best way to do this is not to create such a drawing at all. Typically you create two part numbers
1234-01 BRACKET, RIGHT HAND SIDE
1234-02 BRACKET, LEFT HAND SIDE
Then you detail the -01 on drawing number 1234. Beneath whatever view you would normally label as the part number--usually the isometric view, place this note:
ASME has the note like this instead
But during the brief time that I switched to that to get in line with ASME, I found that everybody was confused by it. Nobody is ever confused by MIRRORED.
If you really have to do it, though, look at Lorono's advice at the bottom of this thread: