This Ableton tutorial walks through the complete workflow of connecting an iPad to Live using Ableton Link, from audio routing to tempo synchronization. The audio output of the iPad is connected to an audio interface input, Link is enabled in the iPad app's preferences, and then activated in Live via the Link button.
Once Live detects other devices on the network, the Link button displays a count of devices available to join the session. A moving progress bar in the Arrangement position reflects the global quantization setting in Live.
To hear the iPad's audio, the track's monitor is set to 'In' in the routing matrix. From there, pressing play on any connected device joins it to the Link session. Any tempo change made on one device propagates instantly to all others in the session.