Just Google "free walking tour" and the city you will visit.

There are NEVER=NIE ´free´-there will always be considerable pressure for a ´donation´ to put in the hat/box/whatever. Major cities will offer several. Most often the ´guide´ has a very clear and easy to distinguish umbrella/hat/coat etc. to recognise on a major place.

BUT: free may be some app that the new fashion on to put all things on the fone, can be downloaded. For that-as above; google remains your best friend. Even tough it often looks that young people have even lost that idea that google could link to answers in a few millisecs

You can always check if the Greeter network is active in the city you want to visit. See: https://internationalgreeter.org

This is a network with locals who want to show you their city. Free of charge!