Once you have dug some soil, the next stage is to dry it out, completely. This is very important! Spread it out in a tray.
Leave it for several days to really dry out. When it is completely dry, you can slake to mush. Add water to your very dry soil (it might help if you break up any big lumps), and let it sit for 12 hours or so. Left undisturbed, it will quietly dissolve. It’s quite strange how this happens, but it really works. You need to really resist stirring it for a few hours, and then the water works its way in and the mud just falls apart. it’s quite strange to see.
Once it has completely dissolved, you can get your hands in there and stir it around thoroughly, making sure it is completely combined with the water. What you have now is essentially a bucket of slip, with some interlopers: some pieces of organic matter, and some sand and silt. The good thing is that they will now reveal themselves!
Let your bucket sit for a short while. You don’t want it to settle too much, but this is a good opportunity to get out any big pieces and to let it become really homogenous.
Next up, you’re going to filter your mixture. You can buy filters that sit in the top of a bucket but while you’re getting started a large sieve will go a long way. I use a 120 mesh sieve from a ceramics shop. Pour the layer of watery clay through the sieve.
It might take a while to go through, and moving it around with your hands is really helpful. A clay particle is something like 0.002 mm across, so you can use a pretty fine mesh.
Once your slip is filtered, you can leave it in a bucket or tray to settle. When it’s had a good while to settle, you’ll see clear water on top, which you can then pour off. Then you can leave it in the sun or a warm place to dry out a bit. Test it every now and again to see what the consistency’s like, and when it’s at the stage you want it to be for working, bag it up.