It looks like they were cut rather thin. Thats not ideal, as you will have bending and cracking during drying.
The most important step is to limit moisture loss through the end grain. You can use a commercial product like anchor seal, or you can paint the ends with latex pain or dip them in wax.
The next step is called stacking and stickering.
https://www.woodmagazine.com/materials-guide/lumber/how-to-succeed-at-air-drying-lumber This is a pretty good guide. You want the wood covered from rain and direct sunlight, but so that the wind can blow through it. You need airflow to dry wood well.
Depending on where you are, if you want to ship me a small piece of the wood I have a pinless moisture meter and could give you a moisture reading to tell you how long to dry it. Otherwise I think about 8 - 10 months should be good for walnut of that thickness.