Planting a pecan orchard
I was contacted by a farmer a few years ago who wanted to plant a pecan orchard on ten acres of his farm land.  He wanted to know if I would be interested in supervising the complete construction of the orchard, from the tree planting to installing an irrigation system.  Even though I eventually determined that I simply did not have the time necessary to assist him, I did spend a considerable amount of time thinking about the whole process.  With the confession that I only almost planted a pecan orchard once upon a time, I offer my thoughts and advise on undertaking such a huge and expensive project.
If you're reading this, there's a good chance that you're still in the planning stages, or even the 'just thinking about it' stage.  I would like to encourage you to consider searching out an established orchard that is for sale before you commit yourself to planting an entire orchard of pecan trees, especially if you are approaching middle age.  Keep in mind that you will not see a single pecan for at least five years, and will not have a sufficient crop to even harvest for close to ten years.  You will not break even on your annual expenses until years twelve to fifteen, not to mention recoup the initial expense of planting your orchard, which may come at year twenty.  If I haven't talked you out of planting an orchard yet, then I remind you that you must commit yourself to care for your young orchard diligently for the long haul.  It pains me to drive throughout the countryside and see abandoned and neglected pecan orchards that somebody lost interest in years before the orchard had a chance to prove itself.

So you still want to plant a pecan orchard
Choose your location wisely.  You must have access to an unlimited supply of water.  Your orchard will not survive without it.  Soil conditions must be considered, along with the type of soil.  Your geographic location must be considered.  Is your area suitable for pecan production?
Once you've settled on location, you need to determine which varieties of pecan trees you want to plant.  You will need at least two varieties, compatible with each other in terms of pollinization.  These varieties must be suitable for your area, and should be able to live up to your expectations.  If you wish to produce big pecans, plant a tree that will deliver.
Determine where you will purchase your trees.  I encourage you to shop around, not only for the best price, but for the best tree at the best price.  Remember a three year old tree will produce two years sooner than a one year old tree.  The price difference may be insignificant when you harvest your first crop two years earlier than you would have had you purchased younger trees.
Finally, plan the layout of your orchard.  This includes your irrigation system, which in my opinion, should be installed prior to planting your trees,  Make a decision as to whether you want to water via drip irrigation or with a sprinkler system.  What do I think is best?  Actually, probably a little of both is optimal.  We use drip irrigation and consistantly have bountiful crops of large, well filled nuts.  However, I've noticed that our trees seem to put on the most growth during rainy summers, and trees located close enough to our yard to receive the benefit of our sprinkler are noticably larger than the other trees in the orchard. 
Decide how far apart you will plant our trees, keeping in mind that in some far away distant future, they will create a shade canopy of fifty feet or more.  The 'nut gurus' have determined that pecan trees must have sufficient sunlight to produce optimally.  Thus, while a distance of thirty feet was once recommended, we are now told we should plant between fifty and eighty feet apart.  I personally would plant forty feet apart which will give you 24 trees per acre
The physical layout begins with yards, if not miles of rope or twine stretched the complete distance of your intended orchard.  With an auger and a measuring tape, begin 'plugging' holes in the earth every forty feet (or whatever you choose.)  I suggest you drill all the holes for your trees before you begin planting keeping a distance of at least four feet from any underground irrigation lines.  Below are two diagrams for orchard planting.  Our orchard is planted using the triangle design, which I personally prefer and it allows an additional 15% more trees per acre than the square design.
All you have left to do is plant your trees.  Sounds simple, huh?  The details involved in planting a pecan orchard are too complex to address on this one page.   If you are seriously considering such a task, I encourage you to visit with area pecan producers and ask for their advice and input.
I will close with this thought...planting and maintaining a pecan orchard is like investing in the stock market.  You have to be in it for the long haul.  You have to be able to invest for years without seeing a return.  You have to really want to be a pecan farmer, that's it in a nutshell.
Typical Orchard Layouts