The process of minting coins at the time would be similar to the Roman method and I assume it would have been normal for any large army to carry the required tools and craftsmen while campaigning to melt down captured material, coinage etc to generate his own coinage to pay the troops. Rather than say pay the troops in coins looted from the Romans bearing the image of Roman emperors and gods.
Metal was heated in a hot fire or furnace. It was either melted into a liquid and poured into molds, or softened and rolled into large sheets, which were then pounded into shape on an anvil. Specialist tools were needed, such as tongs to hold the metal sheets and hammers for all that pounding and flattening. TheCollector.com