Swimmers have to learn to undulate effectively if they want to...

They need to learn the stability necessary to start the wave of undulation.
Here’s a simple set to start that process.
2x [10 seconds on/20 seconds off] wall kick; FAST
2x [10 seconds on/20 seconds off] vertical in streamline holding an object stationary; FAST
50 Choice; 6 FAST UWK each wall; descend 1-4
By holding the buoy in streamline while vertical kicking, we’re giving swimmers a much weaker source of stability and forcing them to kick hard to stay afloat.
This same concept can be applied to any type of set and performed in any way you’d like.
Help them feel the skills, help them apply the skills, help them train the skills.