If you have kept the windows and doors closed during the day, you are already well on your way. But from tomorrow you can really get started.
As soon as the outside temperature is lower than inside (often at night or early in the morning), open all windows and doors against each other. This ensures natural ventilation and air change.
You can do that smartly, especially if you have a house with multiple floors. Then open a window on the shade side of your house and upstairs on the sunny side below. Leave all intermediate doors open. The cold air then pushes the warm air out.
Do you have fans at home? Then put them in front of each other that they ‘blow’ to each other to create extra draft.
If you put a bowl of water with ice cubes in between, the house cools even faster.
Floors, walls or other large surfaces hold the heat for longer. Mop the floor with cold water or hang damp sheets in the doorway to make it cooler.
Do you still have to turn a laundry? Hang the wet wax in different places through the house, for a similar result.
When it gets warmer again, you can also do something to prevent it from warming up. For example, house plants can ensure that it is cooler in the room.
Climbing plants such as ivy or a grape plant against the outside walls ensure that the sun does not shine directly on the walls, so that your house warms up less quickly.
A bit more expensive, but a good solution to keep your house pleasant: insulation. The insulation that keeps the cold out in the winter also keeps warmth out in the summer. Double glazing, roof insulation and insulated outside walls help best.