When you as a customer bring logs to me, after they are sawn you need to take them home and dry the wood before it can be used in your project. Some woods like Eastern Red Cedar (Juniper) will dry very quickly but most local hardwoods will dry at the rate of about one year per inch of thickness. So if you have me cut 2 1/2' thick wood it will take about 2 1/2 years to air-dry. This needs to be done on a flat solid base out in the open where air can flow through the stack of wood. Often people will say that they have a nice metal shed or hot garage to dry the wood in. I say "Fine but you will need to put fans near the stack to blow air through it for the next 2 1/2 years." If placed out in the open running east/west God will dry it for free with His north/south wind. All you need is some old tin or plywood on top to keep rain from soaking down through the stack. A little rain blow in from the sides will not hurt the wood any.
The platform you stack the wood on can be made of wood, metal concrete or whatever so long as it is flat, solid and at least 12" off the ground. I like to use deck blocks from the big box store and place them about 4' apart on the length axis and on the width axis with about 1/4 of the width on the outsides and 2/4 or 1/2 in the center. I then place 2x10's in them and 2x4's on top as if I were building the foundation for a small shed.
The base foundation needs to be as long as your longest boards are and wide enough to accommodate all the wood, but no wider that 6'--over 6' and air-flow will be impeded. The "floor joists " should be about 16" on center. This base shown in the above pictures is 4' wide and 12' long.
To get the base flat you can use a level and level is flat. But if the ground has a bit of slope to it you can build on the slope also. I use a string line for each row of deck blocks. The two string lines MUST be perfectly parallel to each other and then each deck block is set to the string and they should each be on a solid foundation. I like to use sand as a base fill to get them to the correct height. Now put in the 2x10 stringers and then back off about 20' to the side and get down real low and sight the tops as if the 2x10's were winding sticks (if you are not familiar with the use of winding sticks google them and then you will understand.) All the wood used in this project must be very straight as any bow/twist in the boards will cause your stack to be off and any "un-flatness" will transfer to your boards as they dry.
As you add the 2x4's, each of them also become winding sticks. If you will look at the picture immediately above and focus on the top two 2x4's you will notice that the bottom of the farthest one and the top of the next one are perfectly parallel and blend together. This is true for each one. I will put a 2x4 on each end and then back off and sight them as a double check before I start nailing them down
Now you are ready to lay your first layer of boards on the dry-rack. The edges can touch each other and make a solid floor across the entire platform. Do not let them hang over and chances are they will not be a perfect fit so you can then re-space them apart a bit so that the outside edges are straight and in line with the edges of the platform. Next, lay a sticker (I make mine about 1" wide and 3/4" high and as long a my stack is wide. Plywood works but use a hardwood--not pine as pine is more prone to sticker-stain--a shadow in the wood where the sticker was that does not plane out.) The stickers are placed directly over each 2x4 and then another layer of wood is added, then stickers, another layer of wood, then stickers and so on. At the very top add another layer of stickers and then your tin and weight the tin down so it will not blow off. Additional weight is good to help keep the wood from moving.
Below are more pictures of some of my stacks as examples. Notice that the stickers are in a straight line directly above the 2x4's and that the thicker wood is on the bottom. The one inch thick wood will be dry in a year and can be removed while the thicker wood will be left until it is dry. Also, the longest boards are always on the bottom so that everything is supported from below.