See the perspective trick used for the stairs down tile in the pic below:

The stairs down tile is a single flat 6x6 tile comprising a reduced scale floor and distorted wall textures (starting full size at the edge of the board and shrinking to match the scale of the 'floor'.

Easiest would be to shrink standard ground textures into the middle of the tile (see the grey sections below) and place shrinking wall textures in the remaining space. Here's a couple of layouts as prompts (the extra lines represent progressively reduced layers of wall texture; one floor per layer):