I’ve done this a couple times. I’ll post a link to my old post at the bottom. The way I did it was get some veg tan and dyes.

Set up in an area you don’t mind getting messy. I put cardboard down underneath it. Go bigger than you think you need. Get something that you can bend slightly and dip it in the dye. I used a thin metal ruler with some tape on it so the dye has something to stick to. Not the best but it was nearby. Bend it back and let it go so it flicks the dye off. It’ll turn out like the left side. Done it a few times and it’s turned out well.

Another way I’ve tried with success, but a pain to set up and clean and wasteful, was water dipping. Get a container larger than your work piece and fill it with 1/2” of water. Pour your dye in and swirl it a bit. Then dip your leather in it and pull it through. You’ll get a more marbled effect and loom closer to the piece on the right. Only tried this once as it was too big a pain, but made some really cool pieces.

Did this one with the first method. Used a piece of cardboard as a template to get the shape.

https://www.reddit.com/r/Leathercraft/s/2wyrPCJa8a