Use stick pipe dope (not Teflon tape) on gas connection threads. Don't use oil-based compounds.
Check to see that the type of gas you have is suited to the new heater.
Check for the required clearances between the heater and combustible materials (see the heater's owners manual).
Verify that the heater's combustion air needs will be met.
It's a good idea to put a garage-based water heater up on an 18-inch-high (minimum) platform, even though this calls for making plumbing changes. Some codes require it (for electric heaters, too). Make the platform of 2 x 4's and 5/8 inch plywood.
Apply stick pipe dope sparingly to the male threads of the heater's flare adapter, keeping the dope back from the first two threads so it won't get into the gas line. Then it cannot foul the gas controls.
Check for leaks with a dish detergent sloution used on all gas connections you've either made or disturbed (Fig. 11). Never test for gas leaks with a flame. If any leaks are found, turn off the gas right away and fix them.
Read and carefully follow the manufacturer's lighting instructions.
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. Some sizzling is okay with a cold storage tank. That's caused by condensed water dripping onto the hot burner. But if a puddle of water forms under the heater, it's from a leak.
Adjust the temperature control for as low a setting as will provide enough hot water for your largest use (e.g., filling the bath tub). The heater uses more gas to keep the water at a higher temperature in the tank.