I put my griddle on right after I start the burners and warm it up on medium heat with the lid closed, once it hits 400 I know it's probably ready. At that point it depends on what I'm cooking, but medium for my Genesis 3 burner is a good starting point. The griddle (in my experience) spreads heat pretty evenly, but if you need a cooler zone make sure one burner is on the far side and adjust your heat with that one, leaving the other ones on low or even off if need be.
For charcoal grills I use the Weber charcoal trays and put them on one side of the grill usually and do my longer cooking items on that side (potatoes for example) and more delicate things like eggs on the cooler side.
Lastly, I just spray mine with PAM before heating it up and rub it around with a paper towel. If I'm cooking hashbrowns I have a squeeze bottle of oil I use to coat them when on the grill.
I hope that helps and you can get some use out of it!