This is a really interesting video. Caitlin Gregg has some tips for classic skiers trying to master leg positioning, kick and glide with the diagonal stride technique. This is a common issue for skiers and worth mastering during the summer months.
She points out that the tail of the (roller)ski should be up at the end of the push, not flat on the ground, resting on the front wheel only. First error to avoid is lifting the ski at the end of the kick, it should be lifting as the other foot drives forward.
There should be a distinct kick and glide phase. Get the weight over the ski, not a central "safety position". Pause at the end of the glide then kick through, close to the foot setting the ski down just in front of the glide foot then gliding through with a weight transfer onto the new leg. If you set the ski down early you'll get the terrible tail slap on snow.