To clean the inside of a windshield, wipe it with a microfiber towel sprayed with glass cleaner. It might take more than one wiping to get rid of all streaks.
Make sure the microfiber towel is clean. Also, do not get the towel very wet when spraying on the glass cleaner, or it will leave smears.
Step 1: Prepare the towel
Fold the towel and spray a very light mist of glass cleaner on it.
Step 2: Start at the top
Choose a top corner and wipe from the top, down to the bottom.
Step 3: Keep going
Work all the way across the windshield. Turn, refold and spray more cleaner on the towel as needed.