It sounds like your device is already registered in your portal. The custom dashboard you'd like to make can use the device and its dataports already, as long as it's in the same portal.
You can add widgets by clicking on the add widget button in the upper-right corner of your dashboard:
Edit the widgets on your customized dashboard by using the dropdown menu found in the upper-right corner of each widget:
Then, in the editing interface, you can select the dataport you want the widget to be reading from:
Check out our support article on dashboards for some further guidance.
Let us know how it goes! If you make something cool, be sure to post it and brag about it in our Project Showcase!