When your colonies grow bigger and occupy most of the hive, you need to add supers, so that your bees can start stocking honey in them. Timing is everything when adding supers: installing them too early may lead to larval mortality, yet installing them too late fosters swarming. It goes without saying that the timing highly depends on your geographic location and its conditions.
A useful tip is to wait for a « whitened » hive, meaning when bees have started constructing wax on top of the frames. It is then time to add supers.

Preparing for harvest, without a queen excluder

1- Use your smoker at the entrance of the hive and on top of the supers, to warn the bees of your arrival.
2- Remove the supers by pulling it towards you, careful not to crush your bees.
3- Check that the frames are separated enough for bees not to feel the need to construct further. Now you can patiently wait for harvest time!

Preparing harvest, with a queen excluder

1- Place the queen excluder between the hive and the supers, which will prevent the queen from laying eggs in the supers and thus reduces the risk of swarming.
2- You must make sure that the hive/queen excluder/supers are perfectly aligned and stable, not only to make sure the queen excluder is efficient, but also to prevent sacking.