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Look for leaks. Fill the tank the rest of the way. Start the filter. Install your heater on the inside of the tank. Add water dechlorinator. Cycle your tank, preferably for at least a few weeks.

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Fill the tank with about two inches of water, then wait for a half an hour.  If there are any leaks, it's better they show up now, rather than when you have filled the entire thing.  If you don't see leaks, fill up the tank about 1/3 full. Do this somewhere where you won't mind water if it does leak. Have sealant on hand so that you can dry the tank and start fixing it. Once you are sure that all the decorations are just the way you want, fill the tank up to just under the rim of the tank, leaving a gap of 1 inch (2.5 cm). Fill the reservoir of the filter with water, and plug it in!  Water should smoothly (and quietly) circulate after a couple of minutes. Plug in the powerhead/pump if you have an under-gravel filter.  Water should start moving vertically in the lift tube(s).  Wait for an hour or two and check that the temperature is still in the safe range, that there are no leaks, and that the water is circulating properly. It will attach with suction cups. Try to position it near or at the mouth of the filter expelling water. This way the water will be evenly heated.  Most of the thermostats on new heaters now come pre-set at an acceptable temperature range of 70-77 degrees Fahrenheit (21-25 degrees Celsius).  Plug in the heater and install your thermometer. Do not turn it on until the tank is filled completely with water.  The fully-submersible heaters are the easiest to use.  Look for one with an adjustable thermostat, since different fish prefer different temperatures. A good rule of thumb is 3-5 watts of heat per gallon of water. Most fish like it between 70 and 80 degrees Fahrenheit. Basically, keep it between 78-82 degrees F. or 28-32 degrees C in a community tank. Some lights (sometimes included in starter kits) put out so much heat that the water temperature will be drastically altered. When it is turned off, the temp. falls drastically as well. Not good for fish. If this happens, simply go to the hardware store and get the kind that doesn't give off such extreme heat. Remember to turn on the heater only after water is added to the tank. Give the heater time to adjust the temperature before cycling the tank. Tap water contains chlorine and other chemicals that will kill fish, so it's necessary to add a neutralizer unless you use distilled water off the bat. Add the dechlorinator according to the instructions on the bottle. This is also the time to add a starting dose of SafeStart or another bacterial catalyst which will speed up the growth of good bacteria. Make sure to follow the instructions on the bottle closely. The activated carbon of your filter may need to be removed while the chemical circulates, otherwise, the filter may remove it before it has a chance to detoxify the water. For instructions on the fishless cycle (the most humane way to grow the beneficial bacteria all tanks need) see Do a Fishless Cycle.  The cycle must be completed before you add any fish to the tank, or they will die. During the cycle, you need to monitor the water parameters (pH, High pH, Ammonia, Nitrite, and Nitrate). When the numbers for Am., Nitrite, and then Nitrate spike and lower to 0, you have completed your initial Nitrogen Cycle and are in the clear to add fish. (To help move Ammonia and Nitrites along, you may have to use Ammonia Remover. The only way to reduce the Nitrates is to do water changes and physically remove the bad chemicals). Remember to continue doing water tests, especially with a new tank. You may need to do daily 15% water changes to keep your fish tank clean, depending on the nitrate levels. Adding live plants will reduce the number of nitrates as well.