This is how I do a water change. I use buckets, as I don't have a python or one of those automatic water changers.
I have one of those gravel-vac siphons which is basically just a wider rigid tube connected to some hose. I've also got a bucket that I've marked up so I know how much water I've got in it. e.g. there's a halfway mark which = 20 liters. That makes it easy to know how much water you are putting in / taking out.
I switch off the heater and filter. This is important, as if the heater gets exposed, it can shatter.
I stick the wider end of the siphon into the gravel at the bottom of my tank, first checking that there are no fish in the way
then I start the siphon by sucking at the other end, and direct the flow into the bucket.
(As you suck, keep an eye on the hose where it loops over the edge of your tank, the moment the water comes over that, it should start siphoning out by itself. To stop the flow temporarily, pinch the hose.)
Never leave the siphon to work by itself, always keep an eye on it. If you don't, you'll either suck up a fish, or some sand, or you'll drain out too much water. Or the end will slip out the bucket and you'll get water all over the carpet. Guaranteed!
I move the gravel-end of the siphon around, digging it gently into the gravel, careful not to disturb the fish too much. Also if you have sand, it's easy to suck up sand, which is a pain. So keep an eye on it.
To stop the siphon, pinch the hose and lift the mouth of the siphon very slowly out of the water. Release the pinch gently, which will allow the water in the siphon to flow back into the tank. If you do this too quickly, you'll get a rush of water stirring up everything.
When I've removed all the water I plan to, I empty the bucket. This actually takes a few repeats, depending on how much water I'm removing.
Now I fill my bucket with tap water, mixing in the correct amount of dechlorinator (I use prime) and adding a bit of hot water to get the temperature as near as I can to that in the tank. I sometimes use my hand to test the temp, sometimes a digital thermometer.
I've got another, larger bucket with a pump in it, and a hose going up from that pump that I can hook over the edge of my tank. (I don't just drape the hose, I secure it with a hook otherwise the kick from the pump starting jerks the hose off the edge of the tank)
I pour the dechlorinated water into the larger bucket, and switch on the pump which gently fills the tank. No more lifting / siphoning needed.
When the water level is high enough, I switch on the heater and filter again. Easy to forget this step! Also, if you have a hang on back filter like mine, you'll need to prime it (pour a bit of water into it) until it's pumping properly again.
And that's it! Easier to do than to describe.
