Take a look at the current state of your windshield. Windshields that are grittier or scratched will likely need a machine polisher to ‘make them as new’, while mildly damaged windshields may be able to be polished sufficiently with a little elbow grease.
Purchase a glass polishing agent made specifically for windshields. You can use either
(take a look into the eBay Motors Category) or any local auto supply.
Buy or rent a machine polisher if indicated for the condition of your windshield or if indicated by the polishing product you bought. Like at the list item above, you can buy it through or an Hardware Store, by the way orbital polishers have more functions than polishing cars, you could prefer buy than rent. Don´t forget to buy one glass polishing disk.
Read the manufacturer's instructions that come with the glass polishing agent for correct use. Most polishers are applied in a circular pattern over the entire windshield, allowed to dry and then rinsed off with water.
Go over the windshield with a clean microfiber cloth to give it a final buffing and remove any residue there might be. Task Completed, hope you like the result!