Photograph: Matthew Korfhage
Griddle seasoning is, quite simply, protection for your griddle. When a thin layer of oil is heated above its smoke point and allowed to cook off atop the griddle and then cool, it forms a polymer that protects the top of the griddle and leads to better cooks. (Some think that griddle seasoning is to help food not stick to the metal. It may or may not add some nonstick properties, but the main purpose is to protect the griddle against rust and other wear.)
For the first time using a griddle, look at the instructions for each griddle. Some arrive with shipping oil that must be scrubbed off with soap, before seasoning the griddle from scratch. Some, like the Weber Slate Rust-Resistant 3-Burner ($799), come pre-seasoned with food-grade oil and are ready to use immediately once assembled.
But seasoning is not a one-time process. You'll probably want to add a layer once every two or three times you cook, because seasoning wears down over time.
In order to season a grill, I usually prefer to use a plant-based oil with a high smoke point. Grapeseed and avocado are great, but can be expensive. I'm perfectly happy with a big jug of canola oil. Two or three tablespoons of oil is the most you'll need for the entire grill surface.
The easiest method is to use a good pair of grill tongs like my favorite pair from Traeger ($25). Fold up two or three paper towels to the size of a sponge, then lock them into the jaws of the tongs: This is now your oil-spreading rag. Heat up the grill to around 300 degrees Fahrenheit, and then use the paper towel to spread a paper-thin layer of oil across the entire surface of the griddle, including the walls.
When the griddle reaches temperatueres much above 400 degrees Fahrenheit, the griddle will start to smoke. This is good. Wait until it's done smoking, then let the griddle cool a bit. You may have to do this two or three times when you first get a grill, until you have an even, black-brown, seasoned surface.
But later on, a single coat will suffice, as needed—you'll be able to see when the seasoning starts to wear thin in places, but you can also just make a habit of re-seasoning once every few cooks.