Logs are cut and converted to lumber while the log is still wet, commonly referred to as green, and the material needs to be dried before it can be used in your shop. Air drying wood slabs is a really low tech way to dry lumber.
Before stacking the green wood paint the end grain to seal it. End grain is very porous and, if it hasn’t been sealed, moisture can escape from the end grain too quickly, causing the wood to crack. You can use left over latex paint or end grain sealer for this.
MC (moisture content) is expressed as a percentage. 30% MC (not at all unlikely for a freshly cut log) means 30% of the log’s weight is made up of water, 70% is made up of wood. Air drying typically brings MC down to 12-14%. Final moisture content shouldn’t be guessed at. If you’re going to dry wood, you really need to own a moisture meter.
Making the stack
Air needs to be able to flow over each board and throughout the stack. Use stickers to provide space between each layer so you have good airflow. Some woods, like maple, are more susceptible to stains from stickers than others. The safest way to avoid staining is to make the stickers from the same material as you are air drying. Stickers should be spaced about every 16”, and should align with each other vertically to prevent sagging.
A rule of thumb for air drying wood slabs is to allow one year per inch of thickness. Some environments may demand more time. Use a moisture meter to check the material.
Check out this great article for more insight on air drying wood.