If you heater isn't connected to any sort of automation, you'll always control it from the unit itself. Click the on button, and then adjust the temperature to where you want. If the water temperautre is below the set temp the heater should kick on. 

If you have EZ TOUCH automation:

You'll need to activate the heater in heat settings. Click menu, go down to heat, and then select pool. The last line will show where you're getting your heat from or if its on. If it says 'Heat: Off' go down until that line is blinking and use the up arrows until it says heater. Once it says heat: heater, anytime the set temp is below the actual temp the home screen will say 'HEATER' and the heater will be on.   You can adjust the set temperautre of the heater on this screen or the home screen. On the home screen use the left and right arrows to adjust it. You'll see the set temp changing on the top 'POOL' line.

If you have INTELLICENTER automation: 

Click the body of water you want to heat. Make sure the heater is active in the bottom left corner and then adjust the temperature to what you want to heat the water to. If its not active, click heater and then select which heater you want to run. If the set temperature is above the acutal temperature you will see a flame emoji on the home page on top of that body of water.