Measure the backyard. If your backyard measures the NFL required one hundred and twenty-yards by fifty yards, fantastic. But for many people an entire football field will not quite fit. Take a measurement of the area that you'd like to use to construct your field. This will give you the length of the field inclusive of end zones.
Mark your field. Now that you have the measurements, mark your field into twelve equal increments. These will be your yard-lines. Ten, twenty, thirty and so on...including zones.
Spray paint or line your field. Depending on how much of a "weekend warrior" you are, decide how intense you want to get with this process. If you want exact measurements and straight lines, take the time to use a protractor and rope to determine a ninety degree angles for boundaries and yard lines. If it's not as important, "guestimating" is fine and will save you time.
Purchase goal posts. Again, not completely necessary depending on the devotion you have to recreating a NFL football field, but certainly a nice touch when it comes to impressing your friends. Sporting goods stores sell inflatable goal posts that you can place in each end zone to kick "field goals" and "extra points." They are also safe should someone accidentally run into one.
Inflate your football. The pigskin should be firm without being hard.
Determine the rules. When your friends come over to play, determine the rules prior to playing. Some people feel comfortable playing "touch" or "flag" football, while others want the full on NFL tackle style.
Have fun! Now that you've created your own football field in your very own backyard, enjoy it!