Develop the square halt correctly, not by destroying your halt.
Nailing the centreline is not only important for your dressage score, but also for your horses development.
When our horse first pops its head down into a frame it can be a moment of hallelujah - quickly followed by panic as the horses head comes up and we think, 'oh no! I don't know how I did that, I don't know to make it happen again and I don't know how to keep it'.
Nailing the canter transition comes down to getting the "pop" into the canter and communication.
The challenge is in the change of bend.
This is a suppling and balancing exercise, requiring deeper angles of bend and exaggerated shifts in weight and balance.
Using turns and diagonals to improve balance, paces and accuracy.