This is tough cause I can't see the valve setup. I have to assume they are manual valves. Normal operation probably takes some water out of the pool and spa than puts it back into both. To heat the spa, set the intake valves(the ones in front of pump) to take water from only the spa. Set the jets(valve(s) on output side of filter) to send water back to only the spa. The pool will be out of the loop. Normal operation usually has water taken out of pool and spa than put back into both. Make sure to put more water back into spa than you take out, (Adjust valve)