Depends on how deep the scratches are...if minor, I use a windshield clean/polish liquid in a black bottle by Yamaha --sorry guys, but the stuff has been in use since 1981 and it works great removing swirl and minor scratches fine.
Clean the windshield with clear filtered drinking water first to get all the dirt and debris off. Use a clean white cotton tee, use a few squirts, and using circular polishing motions do problem areas first. Let dry to a white haze, and use a another clean white T to wipe and polish, rotating frequently.
Check it out and if the main scratches/swirls are clean, do it again one more time but this time do the entire windshield.