I didn't give kitchen scraps until mine were a couple of months old. I live in a sand pit so giving grit isn't necessary for me. If you are going to free range them at all, place a chunk of sod with the attached dirt from your yard in the brooder. It will give them something to peck on as well as introduce them to the natural grit and organisms in your environment.
A broody hen introduces them to grit as soon as they come off the nest.
You can usually buy chick grit at the feed store, usually. That's just scrap from a granite quarry that has been sifted to get the right size. You can do like a broody hen and just put them on dirt. They will find their own grit. You can give them a coarse construction sand to use as grit. KPGoldstar's way works. Get some sand from a sandbar in a creek or river.
There is nothing magical about grit. Chickens don't have teeth to grind up food. They use small pieces of rock in their gizzard in place of teeth. They'll use any rocks they can find, including larger grains of sand.