Organic soil is one of the cheapest substrates you can use, but you should follow some basic steps when using it.
1. Buy the soil, this is pretty self explanatory, but make sure you get organic soil or else harmful chemicals may dissolve into the water column.
2.Use some sort of material with small holes like a strainer or mesh to sift the larger particles out of the dirt. If this step is not taken the wood will float up and cause slight clouding in the water.
3. (Optional) Pour the sifted dirt into a tank or container and let it sit under water for a few weeks.
4. Use a cap, sand or gravel, to ensure the dirt stays a the bottom of the aquarium, or else the entire tank will become clouded with dirt.
5. Occasionally poke at the substrate to release gases.
You should research a lot more if you ware going to use potting soil. This is just the basics.
This is the soil I chose.
Pile you don't want to use.
In the tank(you can do something like this)