Removing the screens will always give you a better clean. However, you can still try to clean up the windows as best as you can with them still on. Inside, you can vacuum the screens and the bottom to get as much dust out as possible. Clean the screens with warm, soap water. Outside, you can use a spray bottle to saturate the window with your homemade window cleaner through the screen. Let it sit for ten minutes before using a hose to rinse it. Make sure the window is locked. Once done, open the window from the inside to dry off the windowsills and excess waters.