First thing you should do is seal the end grain VERY IMPORTANT. You can do that with end grain sealer that you can buy at woodworking hobby shops, old latex paint, or even wood glue mixed with water 50/50. Just liberally brush on any of these.
As far as drying it there are several solutions. 1) you can air dry it outside. about 1 year for every inch of thickness. should be covered and stacked atleast 6 inches off the ground or better in your garage. 2) You can build a kiln. either passive solar or active with a light bulb (heat source) and a dehumidifier. This method can get you to the months or even weeks time frame. 3) have it kiln dried but is expensive and may take just as long because they wont fire a kiln that is only partialy full. meaning they will wait for other small orders to put with yours until they have a full kiln to fire.
Walnut slower is better. You will get better color in my opinion.
Dependin on how much you plan on doing this you may want to get a moisture meter.
hope this helps
Chris