1. You have to draw a "copper area" on the top copper side. Then, with a right click on that area you can go to the "properties" menu and select the net this area should be connected to. If your ground net in the schematic is called GND, the name GND will appear in the name select box.
2. If you route manually, simply switch to the bottom copper layer and arrange your tracks there. All component pads which are connected to the GND net will be automatically connected to the GND copper area.