For users who already sync Gmail to their email client with IMAP, checking the box is the only step. For those who haven't yet configured their email clients, the Data Liberation team has prepared this step-by-step video:

Google makes an effort to allow users to "liberate" their data, as is clear from the Data Liberation "Front's" website. Google's Brian Fitzpatrick, speaking at OSCON this summer, argued that "if you lock your users in, it makes you complacent." Google's "per-product liberation" approach with Google Takeout contrasts starkly with the fitful data portability efforts by Google's competitors, such as Facebook.

Are you a Gmail user? Do you use a client application for your email, or do you use the Web interface? Let us know in the comments.