Just create your materials, then go into edit mode, you will see in the material properties panel that the assign button is now showing. Choose which faces that you want the material on. Choose the desired materials in the material properties panel and then click assign. If you are weight painting around materials for particle systems, it is more convenient to separate the selection and apply the material separately and then you can rejoin them if need be. Which you can do by right clicking on the faces selected and then go down to separate and click separate by selection. Then if you need to join them go to the object scroll down in object mode and you will find the join option there.