how to summon layered snow in minecraft?

You cannot create a staircase of snow with one command, because each individual block would have different properties (namely the layers property, in Minecraft Java). By using multiple command blocks or a command I do not know, you can make this.
In Java Edition, the command to create a block of snow at 0, 64, 0 is /fill 0 64 0 0 64 0 minecraft:snow[layers:x], where x is how thick you want your snow to be (1-7). Therefore, to create a staircase, you can use a command like that and increment the x or z value in proportion to the variable x in layers. Such a command chain might look like-

/fill 0 64 0 0 64 0 minecraft:snow[layers:1]
/fill 0 64 1 0 64 1 minecraft:snow[layers:2]
/fill 0 64 2 0 64 2 minecraft:snow[layers:3]
/fill 0 64 3 0 64 3 minecraft:snow[layers:4]
/fill 0 64 4 0 64 4 minecraft:snow[layers:5]
/fill 0 64 5 0 64 5 minecraft:snow[layers:6]
/fill 0 64 6 0 64 6 minecraft:snow[layers:7]

which would create a path of snow 7 blocks long from 0, 64, 0 to 0, 64, 6, each block one step higher than the last.