To enable Link, ensure SYNC and Snap to Beatgrid options are enabled under DJ Preferences in the SETUP screen, and your primary Serato hardware is connected - then click the ‘Link’ button that appears near the top left-hand corner of the Serato DJ Pro interface. The ‘Sync’ button in the virtual deck in Serato DJ Pro turns into a ‘LINK’ button when Link is enabled.
Once Link is turned on and a Link compatible second application or device joins the network, a Link session is started.
The first instance of Serato DJ Pro (or other Link application or device) will set the initial tempo for other Link applications or devices, but any tempo changes in the session will change the tempo session-wide.
NOTE: Link also can also be used with the Sampler slots, and will change the respective Sync buttons in those sample slots. More Ableton Live Link specific information can be found via Ableton's Support knowledge base.