First of all, if your pistachios are not already roasted, spread them out on a baking sheet and roast them in a 350 degree F oven for 6-10 minutes, stirring occasionally so they don't burn. Take them out when they are fragrant and look toasted. Let them cool completely.
Place the pistachios in a food processor and process them until they are a smooth paste. At first they will be in fine crumbs and it might take several minutes for them to go from crumbs to a paste, but keep processing. Once you have a paste, stop the processor.