Start them slowly. Free range can mean a lot of different things. I'd start with some supervised free time. Not only supervised for their safety, but realistically you're not going to be able to take your eyes off watching your cute little chicks explore the world
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Open the door an hour or so before their normal bedtime. Understand it may take them a while to explore, they're baby chickens and know they're at the bottom of the food chain. Doing it at this time in the evening will play on their natural instinct to return to their established sleeping place, they should automatically head to bed as dusk nears. You can let them out a little earlier each night.
Combine this with training them to come for scratch. A little bit of grain in a can, shaken, makes a great cue to teach them to come to. Training them to come to a shaker can might come in very handy down the road if you have lose birds. Shake to make the sound, then toss out a bit of scratch so they come to associate the sound with the food. You can also add a call, I do "Chick chick chick". The little buggers are highly motivated by food, and with a week or so of this you'll be able to call them back to the coop during the day if you need.
Keeping them safe from predators is pretty individual. I let my littles out when they're 3-4 weeks old, but it's with the larger birds, my dog to keep an eye on things, and just in my yard/close to my house. I don't have much of a predator issue. We each have to make our own assessment and plan accordingly.