The traditional approach is to sandwich the door bottom between two
pieces of scrap lumber with clamps. Cut through all 3 sections, and
you'll avoid splintering the middle piece.

You also might want to score the cut line with a razor knife.

John Walters wrote:

I have a pair of sliding closet doors that need 1/2" cut off the ends.
These doors are old and I don't want to have to repaint them. What would be
the best way to trim them without splintering?

Thanks,
John Walters