I built one once, but wasn't happy with it and I haven't tried it again. There are many plans here: http://solarcooking.org/plans/
There are a lot of things that will affect how well a solar oven works. Whether it got direct sunlight, how long the food is in it, what type of pan you use and so on. Besides putting foil inside your grill (I'm assuming it's a bowl shape?), put the food in a dark colored pan with a glass lid if you have it, or a dark colored lid otherwise. You may have to move the grill around to keep the sun fully on it, and having it at an angle will heat it better because the sun will hit it more directly.
Now you've inspired me. I have one of those small grills that's never used. Maybe I'll make a solar cooker out of it myself!