The hand signals are pretty simple:

• I'm OK - Tap your helmet with your open hand
• I want to go in that direction - Simply point with both hands in the direction you want to travel
• I want out!  I am having problems - Wave one arm in the air.  This may be a precursor to pulling the quick release buckle so safety downstream are prepared

The LA County life guard training video gives a great demonstration: V-Lower Signaling.

Take the time to review the set of hand signals with your belay team so everyone is on the same page before heading out.  Rapids are very noisy and pretty much impossible to communicate verbally.

As with any complex technique, you should plan for mishaps.  Set safety above and below the rapids.  Ensure the people in the raft know how to trip the quick release knot.  Everyone in the raft must be SWR trained and know what to do if the raft capsizes.