Start out with:
(require 'multiple-cursors)
Then you have to set up your keybindings - multiple-cursors doesn't presume to know how you'd like them laid out. Here are some examples:
When you have an active region that spans multiple lines, the following will add a cursor to each line:
(global-set-key (kbd "C-S-c C-S-c") 'mc/edit-lines)
When you want to add multiple cursors not based on continuous lines, but based on keywords in the buffer, use:
(global-set-key (kbd "C->") 'mc/mark-next-like-this)
(global-set-key (kbd "C-<") 'mc/mark-previous-like-this)
(global-set-key (kbd "C-c C-<") 'mc/mark-all-like-this)
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