I just figured out how to SHDL yesterday and realized what I was doing wrong for so long.
I assume you play standard grip (not claw) so you should observe how you actually do the Y/X -> B movement and where you start the jump input on the button itself.
I was told going from X -> B is really simple because you can press A during jumpsquat and it won't do a nair. So you can just drag your thumb across to B which makes it easy.
I did that for a bit and was decently consistent. It taught me the timing itself and how strict it is. Its much easier than what I learned for Y -> B and tapjump SHDL. But it was still was awkward cause I used Y to jump and JC.
What I actually learned was the gross motor movement of my thumb.
For the longest time I tried to use Y and flick down from the left end of the Y button.
Last night I discovered you can go from the right end of the Y button to B and do a movement with the whole thumb.
Ultimately its a similar movement to X -> B
Personally I can produce a more consistent movement with my whole thumb than just the final joint in the thumb.
My SHDL guide illustration:
So I made this thanks to the vector art from wikipedia.
Yleft -> B is the shortest path, Yright -> B (the middle blue line) being the medium length path and X -> B is the longest path. You're given X amount of frames to start the first lazer after inputing the jump.
The shortest path might look the best on paper but when you consider where you place your thumb and how fast you can safely accelerate, the other paths become better.
With Yleft theres no horizontal movement so you'll be relying on the final joint movement which can be inconsistent and tiring.
With the other options you can use the whole thumb to make a rolling motion, a draging motion or a compound movement which most of the time its easier.
I hope this makes it easier for other people how to learn SHDL. These are just the different ways you can SHDL. If you practice the paths from right to left you will get faster at it naturally. I know this is silly but text answers don't really explain it. We spend a lot of time talking about inputs and the number of frames we're given to execute inputs but we neglect how to actually do the input. Not just which input option but the physical movement itself. I hope this helps.