Here is the plan:
Now you want to work as quickly as you can, so you want to have
everything thought out and gathered together before you start. Have
your new gravel rinsed and ready first, for example.
You'll need several buckets or plastic containers of some sort
that have never been used for anything except fish. They need to
have never had any type of cleaner of chemical in them. One of them
can be an ice chest. Large Tupperware type containers work also.
For this article, I'm going with an ice chest because it will help
keep your fish at a steady temperature.
A Python is very, very helpful with this whole process, and the
article assumes you are using one.

Click picture to order your Python now.
Drain some tank water into the ice chest, and place fish in the
ice chest.
Turn off all filters and heaters.
Drain some tank water into a bucket. Use this bucket to clean your
filter media, such as your bio wheel (if you have one, or floss,
whatever non-replaceable media you have). Just swish it around in
there. Then, put the bio wheel (or floss) in the ice chest water
with the fish. Close the ice chest and put it somewhere out of the
way where the temperature won't change much. Like, not outside or
in the garage.
Pour out the tank water you swished the bio wheel in. Pour more
tank water in the bucket and clean the rest of your filter media
in it, like the blue pads or what have you. Swish them around to
clean them as best you can. If they are wearing out and need replaced,
don't worry about this step so much, but, keep these pads wet for
now too. Do you have real plants? If so, carefully remove them,
and put them in the bucket too, they'll need to stay warm and wet.
Set that bucket aside.
Drain the rest of the water outside, or down the drain, while vacuuming
the gravel. You are going to get this gravel out, but you might
as well have it as clean as possible now, to save you work later.
Remove all your decorations. You can clean these in tap water,
because you are protecting your bio-colony in your filter media
you have in the ice chest. Clean your filter casings now too. (The
casings, not the media!)
If you are going to keep the gravel to use in another tank someday,
you'll want to remove the gravel and put it in another bucket or
buckets. Set it aside for now.
Now, your tank is empty, right? Take it outside with your algae
scrubber. Use the hose and the scrubber to clean it up. Or, if it's
cold out, you might be able to use the bathtub if the tank is not
too large. Be careful, it'll break if you hit it on the side of
the tub. Outside is better, but if it's a large tank, clean it right
where it is. Scrub that sucker clean. Take it back in the house.
Put the new, already clean and rinsed gravel in. Put your newly
cleaned decorations back in. Put the dechlorinator, such as Prime,
in and a couple inches of water. Next, put your plants back in.
Do all your aquascaping now.
Fill the tank back up slowly and gently so you don't raise too
much dust. Make sure the water is the proper temperature coming
out of the tap. Make sure the heater is in place and plugged in,
after the water level is high enough. Put the newly cleaned filter
back together and get it running. (If you are replacing media like
blue pads, try to keep some of the old media to add in there. Even
though you have the bio-wheel or floss, since you are doing a major
overhaul, the extra help is a good idea.) It should clear the tank
out pretty fast.
Acclimate your fish to the new tank like you would if they were
new from the store, because it's all new water to them.
Sit back and enjoy all your hard work.
Remember to clean your old gravel, if you are saving it, and put
it somewhere that it will dry completely. Then, after it's dry,
you'll need to think of a way to store it. Just don't store it wet
or dirty, you'll regret it when you try to use it again some day.
If you put it away dirty or wet, it can get really gross with methane,
hydrogen sulfide
think "swamp" here.
For example, I was once given a tank that belonged to a man who
had died two years previously. His tank was broken down and stored
by non-fish people. It was stored for two years, and then the whole
thing was given to me. The substrate was just plain gravel ... natural
pebbles ... and it had been stored in zip top bags, wet and dirty.
Two years later when I opened those bags, it was a smelly, gross,
slimy, moldy mess. I'm a stubborn wench, and I did get it all clean
and reuse it, it's still in the tank to this day, but opening those
bags is one of my most unpleasant fish keeping memories!
It was actually easy to clean, but smelly and time consuming. I
just put it in buckets and kept rinsing it with fresh water, and
airing it out in the sun on flat trays. The repeated cleaning and
airing (hot summer sun) cleaned it up well!
Of course, this happened long, long ago, when I was just a fish
keeping noob myself. I had no idea then how cheap it would have
been just to replace it. An even cheaper solution though is cleaning
it up and storing it properly in the first place, to reuse at a
later time.