How Do I Make A Tileset?

Hi!

I'm trying to make 32 x 32 tiles for my game. I have no flipping clue how to do this. I've tried to search everywhere but I haven't found a simple explanation for how to format my tiles.

I want to make my tileset like the girl does in this video:https://youtu.be/jEWFSv3ivTg?si=pgTc4iE5kWOsLua2 so that I can do the whole dual grid system thing. But there isn't a tutorial or anything on how to format the tiles. The one photo she shows of the tileset shows that the tiles on the outside are only half the length of the rest? I just need an example of someone who followed this format and what their 15-tileset looked like so I can understand. My brain is going to explode how do you guys do this stuff help

She focuses more on the coding aspects than the art in the video. I'm just asking about the art I don't need any coding tips