Once you find a portal that is either uncontrolled or controlled by your faction but not fully deployed to you are ready to deploy some resonators. Once you target the portal you can simply click on the deploy button. The deploy button will automatically deploy to at the first open slot the lowest level resonator that you have. If you just want to spam and create a low level portal with your lowest level resonators then this is the best idea as it will be the easiest way to fully deploy to the portal.
While this is the quickest way to fully deploy to a portal it is not the best. The best way is to target the portal and then go to the upgrade menu and then select your resonators that you want to use individually so that you can create the highest level portal without wasting resonators.
Note: The closer you are to a portal while deploying resonators the closer the resonators will be to that portal. This will make it easier for a portal to be attacked. When deploying or upgrading resonators it is best to stand with the portal on the edge of your action circle.