The easiest way to get wrinkles out of a flag, especially a US Nylon flag is with an iron. USA Flag Co. recommends spraying some water lightly on the flag, setting the iron to cool to warm (make sure the iron is not too hot so as not to melt the material) and just gently go over the flag until the wrinkles are out. If you have a steamer, that will also work as long as it is set to a low temperature.