Plug in the hot glue gun and insert a glue stick.
Open the umbrella and cut off the handle using a hacksaw or tin snips. Cut off as much of the handle as possible to reduce interference with the light fixture.
Cut four equal lengths of string or fishing line about 2-feet long. Tie one end of each string to a rib in the open umbrella so that they are spaced equally around the inside perimeter of the umbrella.
Glue the ends of the string to the side of the light fixture while maintaining equal spacing between the strings. Adjust the lengths of the string to position the umbrella. It may be possible to tie the ends of the string to the fixture depending on its type.