I used 2 methods. One from my cast iron experience and one from my wok experience.
First clean the griddle. Soap, scrub, then pour boiling water over it till it's perfectly clean.
Crank it up to high and get it bone dry. I took some paper towel and held it with a long set of tongs. I used Ghee, you can use coconut oil or Flax Seed oil (if you can find it) and if you can't find any of that, go for vegetable oil. The goal is to find the oil with the highest smoke point. So once it's baked on, it can take the most heat.
So with the griddle hot as hell and still getting hotter, I throw a significant amount of oil on the griddle and start rubbing it around the metal. Being very sure to get every piece of exposed metal (including the back wall and the outside of the wall.) The oil will smoke like fire. This is good. It's going past the smoke point and laying down that nice nonstick surface.
This process takes every bit of 30 minutes to do right. Just keep a thin (as thin as possible) layer of oil over the hot griddle and let it smoke. You really need to be careful not to let any oil puddle up during this. We want a thin consistent layer of oil over the entire surface. I went through a lot of paper towel during the process. Put oil down, then wipe it up and spread it around. It will start turning black in the middle. Once you have a nice looking blackish surface let it cool down and wipe up the residual oil layer. Let it cool COMPLETELY.
Then come back for round 2. Preheat the griddle (don't add oil yet) to a little over low heat. Chop up a bunch of onions and throw them on the hot griddle. Then put some oil on the onions and start sauteeing them. Move them all over the grill with plenty of oil. Rub them all over the surface. Everywhere. Let them go till they are black and nasty. I took at least another half hour to do this. We want to make sure there is no metallic flavor left and finish up the seasoning. These onions get tossed and we are ready to make real food!!!
I wanted to really seal in this seasoning layer, so I started with lots of bacon. The bacon grease just adds to the layer and off I went. Mine is working like I've had it for years even though I just assembled it last night.
Hope this helps, I have done a lot of research on seasoning techniques when I went on a Griswold shopping spree!