I remember my mother marking the eggs and letting the hen set but can’t remember what to do leading up to this so all the chicks hatch at once!
You can collect the eggs each day, and put them in the house or cellar (not refrigerator, not incubator) until a hen goes broody. Use a pencil to write the date on each egg so you can easily tell how old they are. (For hatching it's best if they are no more than one week old when the hen starts to set. For eating, they're good a while longer than that.)
Leave a few fake eggs in the nestbox, so a hen has something to sit on if she wants to--no danger of fake eggs rotting if you just leave them for months
When a hen goes broody, take away the fake eggs and give her real ones. If you put them all under her at the same time, they should all hatch about the same time.
Do not give her any eggs on other days, and don't let other hens add eggs to the nest (either pen the broody away from the others, or check each day to take out any newly-laid eggs.) Eggs that are added later would also hatch later, and you're right that you don't want that.
(Recognizing a broody hen: on the nest all day and all night, but comes off briefly to eat and drink and maybe have a quick dust bath, then goes back to the nest.)
