Install Gas water heater
Install a 1/2 inch male flare adapter into the inlet opening of the heater's gas valve. Connect the gas flex-connector collar to the flare adapter and tighten with an adjustable open-end wrench. Check to see that the thermostat is in the off position. Now you can turn the gas on.
Check for leaks with a dish detergent solution on all gas connections you've either made or disturbed. Never test for gas leaks with a flame. If any leaks are found, turn off the gas right away and fix them.
Now you can turn the gas on. Read and carefully follow the manufacturer's lighting instructions to light the pilot light. It may take some time for air to be purged from the gas lines, and a flame should be kept at the pilot orifice until the pilot lights. See that the main burner flame settles down, does not burn yellow but basically bluish, and doesn't smoke.