In the Northwest, where people don't normally have air conditioners, people are being exposed to temperatures that are higher than normal these days. People's first instinct seems to be to open the windows when they are hot. However this is often not the best thing to do.
The first thing you need to do is buy a thermometer for inside of your house. Now if you ever feel hot, look at your phone and see what the outdoor temperature is versus the indoor thermometer that you have. If the outdoor temperature is higher than the indoor temperature, do not open your windows, this just lets hot air in the house.
You can buy a fan for pretty cheap off Amazon, try to use that because what you're hoping for with the windows being open is a nice breeze, which you can get with the fan without increasing the inside temperature.
In my experience the outside temperature doesn't often fall below the inside temperature until about 10:00 p.m. at this point you want to open all the windows in your house for the night, and get your house as cool as possible before the next day. If you have the weather app on your phone, you can usually see an hourly breakdown of temperatures which will help you time everything properly.
Sometimes in the morning usually somewhere between 5:00 a.m. and 7:00 a.m., it will reach the lowest temperature of the day. This is the time when you want to close all of your windows, to seal all that cold air in your house.
Try to keep blinds down, as light can enter the windows and turn into heat but heat cannot escape through the glass itself. Keeping water in the refrigerator ensures a nice cool beverage later in the day. Also if possible try to hang out on the lower floors, or the basement as heat rises and the bottom of your house is going to be the coolest part of it.
I would also recommend taking showers later in the day when it starts to heat up, and if you've ever thought about taking cold showers this is probably the time. There's lots of health benefits to them, and it's really nice to walk out of a freezing cold shower into a warm house. Also avoiding alcohol, even a cold beer, is good as that raises your body temperature.
Turn your power saving settings on your computer so it automatically shuts off within 5 to 10 minutes. Not only are you going to save some electricity like this, but having your computer running all the time is like having a little heater running. Those heatsinks on your CPU and GPU aren't just siphoning off heat into the ether, it's going into your house.