Hi - firstly don't worry - my chickens will either wait patiently until their favourite nest is free or get right on in their with their flock mate to lay - bonkers!
Firstly, you need to plan how you are going to keep a nest and a broody hen separate from the main flock, but ideally still in the coop. I don't separate my broody hen from the rest of the flock, but she is the alpha hen and no other bird would dare try and lay in her nest - you may not have the same situation. Alternatively, you would have to remove the non-marked eggs on a daily basis.
Then you need to ensure that you collect, date mark and store the eggs that you wish to hatch. These eggs should be no more than 10 days old, but i go for 7 days personally.
Once you have a broody hen, test her commitment to being broody by ensuring that she sits on a nest (only getting off it to eat / drink) for at least 48 hours.
Once you see evidence of this, you can move the hen to the separate brooding area or leave her to brood in the nest of her choosing.
I'd strongly suggest that you read up on these topics, as i have only touched on some of the main ones.
All the best
CT