To access the bulk collection editor click the bulk editor button from My Images:
Select the items you wish to edit by clicking the checkboxes to the left
Then select the collections you want to edit from the options at the bottom. If you select the option to be able to fine-tune you will do this in the next step.
You will be shown each step as you go through the editor - no collections will be changed until you click the "Save All " button in the final stage.
This short tutorial will explain in detail how to use this very handy tool: