Place a water bottle in a brewing stand
Add Nether Wart to make an Awkward Potion
Add a Spider Eye to make a Potion of Poison
Add a Fermented Spider Eye to turn it into a Potion of Weakness
Add Gunpowder to make it throwable
Throw the splash potion at dirt or clay to get mud if using a mod or datapack that adds mud crafting
In vanilla Minecraft, mud cannot be crafted directly from dirt or clay