1. Build a structure like pic1.png.
2. Keep walking straight to the corner in front of the upper slab.
3. Press the shift key down and up quickly (need to keep on holding down the W key).

For more detailed operations that I may not know how to describe, please refer to the video (vid.mp4).

After the operations, sometimes the player may get stuck at the edge of the slab, After switching to the third person view, you will see the player repeatedly playing the sneaking animation.

Updated: I am going to show a way to reproduce it with command. It precisely changes the player's coordinates, allowing it to be located exactly at the edge of the upper slab.

1. /fill ~1 ~ ~1 ~1 ~1 ~2 minecraft:glass

2. /fill ~-1 ~ ~1 ~-1 ~1 ~2 minecraft:glass

3. /setblock ~ ~1 ~2 stone_slab[type=top]

4. /setblock ~ ~2 ~2 minecraft:command_block

5. Set the command in the command block to:

   tp [username] ~ ~-2 ~-0.7999999 

6. Activate this command block.
7. Try to move towards the direction of the slab (don't press the Shift key).
8. Then you will see the player was stuck like in the video:reproduce with command.mp4
9. If the player didn't stuck at the edge of the slab (the player didn't try to sneak or the player successfully sneak down), change the number of 9s in the command of the command block and have some other tries.