I've been advised by a vet not to deworm my chickens until they were at least 5 weeks old. The only dewormer I could get was for ostriches
and the dosage was per kilogram body weight. So we had to estimate the combined weight of our flock, measure the amount of dewormer and I mixed it with their water and filled all their water bowls first thing in the morning, when they all have a drink. It isn't always possible to tell if your chickens have worms, so it's a good idea to deworm regularly.
Make sure you follow up with another dose after a week, to kill the eggs and then deworm again every couple of months, just to be on the safe side.
He also gave me a tip for medicating chickens: take their food and water away for 3 hours, then replace with medicated water/food. It works well for medicine that needs to be ingested within a certain time after mixing it with their water.