I purchased new Nevco game timers, can you help me determine some rules or guidelines for installation and layout on a football field?

Typically, they are installed on the end of the field behind the end zone on opposing corners.  There are no rules governing the installation location for play clocks on a Football or Lacrosse field.

Nevco recommends that the clocks be installed in the optimal field of view for the players avoiding blockage from the goal post and any other obstructions.

The diagram below shows the optimal area for such placement.