They need to start laying first. What you can do to encourage them to go broody is:
1. Make sure their coop and run are very secure from predators and that dogs and cats cannot enter.
2. Collect the first few eggs which will probably be very small.
3. Depending on your family's need for eggs, if you have enough already, get lazy about collecting the eggs. Don't watch too closely for a week or so.
4. If you see something like this, check a few hours later to make sure she is still there and wasn't just laying an egg. If she's still there, especially at night, you've got a broody! Mark your calendar for 21 days, but allow at least 25 days, because the hatch can be slow in cooler weather.
5. If all else fails, you can buy either or both of two things: a game hen and/or an incubator. Game hens are excellent mothers and go broody at the drop of a hat. An incubator is good to have, even if you have broodies in case one of them gives up on her eggs.