Step 1: This Is What You Will Need
A net! (for various cleaning needs)
Water changer tube siphon thingy! (the thing wrapped around the bottle of chemicals! ^-^)
Pitcher (not pictured)
Water conditioner and other needed chemicals!
Thermometer (not pictured)
Step 2: Siphon Out That Old Water!
Once you have a constant flow of water going from the tank into the bucket you're going to want to shove the Big end into the gravel, or to use the proper lingo, the "substrate." then pull it out and let all the gravel fall out of the tube. Then move to a different section. Keep moving around until you have either sucked on all of the gravel or sucked out 30% of the water. In the picture i have sucked out around that 30%. DO NOT GO OVER THIS AMOUNT! if you do you will risk stressing out your fish. if you're doing it right you will see dirty nasty water going up the tube and into the bucket! (duhh)
Step 3: Get Some Fresh Water!
Go get the bucket and rinse it out in the sink!
Grab your chemicals and measure out the correct amount for your size tank. ALWAYS USE A WATER CONDITIONER!! i don't care if you have tap or city water! Use it! Pour your chemicals in your now clean bucket.
Then, Grab that thermometer and adjust the water temp coming out of the tap to plus or minus 2 degrees of the temp of the water in your tank. You dont wanna put your poor fishies through temp shock! :O (the ideal temp of a tropical fish tank is around...eh...76-78 degrees fairenheit) Then you just fill up the bucket!
Step 4: Top Off Your Tank!
Don't pour the water in to your tank too fast. Nobody wants those pretty little decorations floating around!
Don't pour right out of the bucket. go ahead and try to do it. you'll see why you don't want to soon enough!