So, take this suggestions for what it's worth. I can say that my newest method of seasoning worked far better than anything else I tried. You will need at least 3 jack stands or two and a third heat proof support - a weed burner and a propane tank.
Scrub/clean griddle as directed from factory. You might wish to smooth up the surface a bit with a sanding disk on an angle grinder - the smoother start the better however this step is optional.
Coat the cooking surface and side walls with season oil of choice (lard) and wipe it down to leave a thin layer only. Turn the griddle surface upside down and place on your jack stand support or other method of elevating the griddle off the ground. Using your weed burner you can then sweep back and forth - heating as evenly as possible - over the entire backside surface. The seasoning should begin to smoke and keep at it until it's pretty much stopped smoking. Be sure to work the corners and edges as they will take a little more heat to get the smoking going. If you have an IR thermometer it helps to monitor the temps across the surface. Note - I always look for an excuse to use my toys.
Let cool completely and begin the process again - repeat as many times as you like. This will get the entire surface (not just the unit burner area) seasoned much faster and with less gas. Obviously keep kids and pets away from the hot griddle. Also, whether or not you use this method I recommend you check to see the griddle surface is not warped from the factory - it happens sometimes.