To be honest, no one ever told me either, they just said to do it and so I did, and when I first started teaching I too just told my students to do it and yep they struggle and I just kept telling them it will get better in time and for the most part it really did; except for those few that really needed more detail on how to actually transition.
This transition is a transition of core work combine with flexibility, but also the right understanding of how to move from point A to point B. When looking at the cliff notes picture below take note from the foot to the hand and practice your transition. Speed is not the issue and usually will only make things more complicated. If your teacher doesn't ever slow things down for the people in the room who need more help in fear their flow may be ruined, then I want to encourage you to practice at home and then go and WOW your teacher at class when you finally get it (or if you are a teacher slow down, your students will appreciate it and feel the difference).
And one more thing, don't forget that this transition is a core move, so work to engage your pelvic floor (really important) as well as your deep transversus (it will feel like your belly) as you roll your body forward, rather than stiffly like a board move your body forward. Remember make room for your leg to come forward as you slide along.
This can be a move in itself and I often use it as that in the event of practicing such a transition for a better Sun Sal or whatever we are doing at the time. I like to rock this ten times on each side to really feel the difference!
So off you go, ready, get set, rock your core!