1. Download Gallery Assist from http://drupal.org/project/gallery_assist.
  2. Move the unpacked Gallery Assist folder (gallery_assist) into the same location as your other contributed modules.
  3. Go to Administer > Modules and search in the list for the Gallery Assist entry. Check the checkbox to activate the module and then save the settings.

The folder “/sites/default/files/gallery_assist” will be created. Gallery Assist also creates folders for any users with permissions allowing them to use the module. Folders named after their UID will be created in the “/sites/default/files/gallery_assist” folder.

I recommend creating a test gallery after installation and before configuring the module. This will give you an idea of how the default settings look.