See also article Groundplane
An easy and quick way to generate a groundplane over the complete board e.g. on the bottom side of the board can be found by the use of the groundplane assistant. Find it in TARGET 3001! by the icons and or in Layoutmenu Actions / Generate Ground Plane. First condition again is the fact that all signals have to be routed (the GND signal too). So you might have a situation like this (black is the background of the screen):
After activating the Ground plane assistant, you first get the following dialog:
What you are going to do if you follow the dialog is to create a ground plane upon layer 2=copper bottom, connect signals GND to it and leave spacings (auras) to all other signals of 10 mil. At the same time you will have a spacing of 20 mil around the outer edge of the board.
After click on the OK button get the following result:
Now see that the complete GND signal has no aura to have it connected to GND:
Remember that layer 1=deletion bottom is touched automatically because all signals are initially provided with a 10 mil aura which is displayed on layer 1. The aura of signals can be edited at any time.
By the way - it is possible to design several planes on one copper layer having different potentials. See the article Several freehand groundplanes. Use more than one figures and assign different signals (potentials) to the figures...