Position the water heater directly under the fittings. You do not want to stress the pipes, so make sure your measurements were fairly accurate if you had to cut any pipe. Make sure the half of the fittings that is screwed into the heater are snug and then slip the washer in between the other half (soldered to the pipe) and tighten the nut.

Make sure you do not get the threads started wrong. You want to be able to tighten this nut down because it will give you another place for a leak if you don't. Tighten both sides, hot and cold. If you're tight enough, there will be no play in the fitting.

Turning The Water Back On

Before you turn the gas supply back on, turn your water valve back on. You'll hear water start to fill the tank. Look for leaks. Hopefully you have none. If you do, pray that they are only because you need to tighten the washers more. If you have to resolder, I pity you. It isn't fun.

If you have no leaks, congratulations! Now let's get ready to turn the gas back on and fire up your new natural gas heater.