The following video shows and explains the transition into Chaturanga from Plank pose. There are many important cues when it comes to practicing Chaturanga: Protract the shoulders, draw shoulders down away from the ears, externally rotate the arms, squeeze elbows in. On additional key concept here is that you need to move the body forward into Chaturanga, and not just lower straight down from plank. It’s a difficult transition to do well with good alignment, so take time with it. I hope it’s helpful and let me know how it goes!