Ideally, dig a hole about 1 meter deep and 2 meters in diameter, either by a lake, in a meadow, or on your own property. The soil should be earthy; otherwise, it won't work. Clay soil is ideal. If you have sandy soil, you'll unfortunately need to get some clay to fill the hole.

Then, thoroughly sift the excavated soil and remove all stones and roots. You won't hurt yourself, will you? Leave the sifted soil around the hole.

Then, fill the hole with water up to the rim and let it soak in.

Once it has all soaked in, fill the hole with water up to the rim again and let it soak in once more.

Once that has also soaked in, fill the hole with water up to two-thirds full again. Now add all the sifted soil to the water in the hole and mix everything well. Perhaps you have someone who can use their whole body to mix the soil and water in the hole, or you can use a shovel or a long stick until it's all like a thick slurry. Your bath is ready.

The purpose of filling and letting the water soak in twice is to saturate all the soil in the hole. This creates a very rich mud. It's a good idea to attach a rope to a tree or something similar at the edge in case you get stuck. It's best to block off the hole so no one falls in. Have fun!

You do realise that water usually just drains away don't you? Have you even tried out your idea?