Changing Table Structures - Mastering phpMyAdmin 2.11 for Effective MySQL Management - page 104

We click the Edit link (small pencil) on the line containing the author_id index; this
brings us to the following panel, which shows the current state of this index:
Next, we choose Add to index 1 column(s) ; we then click Go . We select the language
field on the next panel. This time we do not have to enter a size since the whole field
will be used in the index: