Ground and power will be the most common and repeated connections in your design.
To add a ground flag, simply hit 'G' on your keyboard and click the canvas to place the flag. You can also add a GND flag fro m the Connections Marker tool.
The GND flag will create a net called 'GND' and it will be connected to all other net segments with the same name. In other words, it behaves like all other indirect connections.
Check out our video on Using Flags for more details on using flags in general.
Power connections can be made by adding a VCC flag. You can rename the flag to reflect the particular power net. Check out our Schematic Style Guide for a suggested naming convention for power nets that is clear and readable.