On the community tanks,and divided Betta tanks, I use a Python, so it goes like this...
Unplug the heater and filter
Siphon out 25-50%, cleaning the substrate as I go
Add water, that's about the same temperature, along with decholorinator
Clean the filters off in old tank water
If I have plastic plants in the tank, I take them out and scrub them
And then start everything up.
On the smaller Betta tank I...
Take some of the tank water, and put the Betta in there.
Take out almost all of the water
Put the water back, and add decholorinator
Clean the Plastic Plants
And put the Betta back in, along with some of the tank water he was in.