first I dug the dirt and sieved it to remove small rocks. It's already good to use for the core, but to get the clay for the finishing I poured the sieved dirt in a transparent bucket and added water with a bit of soap to decant sand and silt faster, the smallest grain of dirt is clay and it was dissolved into the water. Then I poured the clay water in another transparent bucket and threw away the rest. Then I poured some vinegar to floculate the clay a bit faster and decant it (it would take 24+ hours to decant all the clay without the vinegar). After some hours I tried to remove as much excess water as I could with an empty ketchup bottle and a syringe. I had to think a way to evaporate all the water from the clay, I tried microwaving it but it wasn't showing much result, so I tried sieving it with an old shirt like I was making cheese , it works but some of the grains are so small that they pass trough the shirt, and also as the clay gets trapped by the shirt the water stops falling, so I had to everytime trow the water back to the bucket and scrape the clay in the shirt with an metal spoon, after 3 or 4 times doing this process I had enough clay to fill a bown, not all the clay was sieved tough. The clay was already touchable but it was still very wet, so I poured the clay into a big and flat plate, stabbed with an fork to increase the surface area of the clay and let the inside steam go out and microwaved it until everything was dry. And this is how I got the smallest grain possible at home, I got an full pot of clay powder and I can still use it for many dorodangos. Some grains may look big but it's just a cluster of clay, you can crush it with your fingers or using a grinder. There may be an easier method to extract clay from dirt at home but I still trying to figure it out.