If you are like me and do not have an aquarium system with an auto drain facility then you will need to siphon the water out manually.
This actually is very simple.
What I do is use some 1″ wide or so aquarium tubing and attach one end to a piece of wood which is just longer than the depth of the aquarium. This allows you to get right down to the bottom of the tank with the pipe without the hose kinking.
I personally use tie wraps to attach the tube to the wood but it is personal preference really. Just make sure that you dont use anything with any metal in it.
To start the siphon get a bucket or some other tool ready to capture the water and then place the end which is attached to the wood into the water.
Now take the other end and place it into your mouth. Be as close to the bucket as possible and suck gently on the tube. This sucking motion will cause the water to be pulled into the pipe. Keep one eye on the pipe and as soon as you see water rushing down the tube get the tube out of your mouth and into the bucket otherwise you are going to get a mouthful of saltwater!
Once the siphon is going then it will not stop until you stop it (or you run out of water!) The water being pulled down the pipe into the bucket by gravity cause the water behind it to be pulled along.
Once the water change is done then simply break the siphon by lifting out the end attached to the wood out of the water.
You can pause the siphon at end time by placing your thumb over the intake, when you are ready to start it again simply remove your thumb.
There are some people who even install taps into the pipework so that they can control the amount of flow going through the tube or they can use it to temporarily pause the siphon.
Whichever method you use remember to watch the tube and as soon as you see water take it out of your mouth because as said if you dont get it out in time you will end up with a mouthfull of salty water.