Have a cage with food and water at the ready. It's best if you hang the cage to the side of a barn or the side of the coop or somewhere that is protected from the rain.
Let mom set and hatch all the eggs that will hatch. This may take up to 36 hours or so.
If mom is a really good broody, she will stay with the chicks and you can maybe leave them alone, if your coop and run are completely safe from predators and bad weather. However, this doesn't always work out...
If mom gets down off the nest prematurely, you will need to sprint into action. You may hear loud peeping from the abandoned chicks.
Catch mom and put her in the cage. She will be really angry, of course. Then run, and catch all the chicks, including any that she left in the nest, and put them in with mom! Mom will be happy again!!
Picture isn't great, but it shows our fourth broody with her chicks in a cage. She flew down with two chicks and left two up in the nest plus a pipped egg. I caught all the chicks and put them with her in the cage. Broody hens aren't always totally successful, but they save a great deal on the electricity costs associated with running and incubator and then a brooder.