You want to get some nice blackout thermal curtains for any windows facing south. You might actually need 2 sets of curtains to fully block the sun. I’ve noticed a big difference this year since I added a 2nd thermal curtain on top of the 1 I had. Unfortunately that only helps so much and as long as it’s unbearably hot outside you’re going to see the heat increase inside as the day goes on. At night open all of your windows and run fans to get as much airflow as possible while it’s cooler outside. In the morning close your windows the minute the outside temperature is the same as the inside temperature. As long as it’s cooler outside, the windows should be open.