Remove the turtles and get a clean rag or sponge. Clean the internals of the tank. It's important not to use chlorinated water or chemicals to clean the tank or you will kill the good kinds of bacteria that help to keep the water clear.
Clean the tank. Throw away the filter's fine particle media and replace with new. With dechlorinated water only, very lightly rinse the biological media. Don't worry about getting it clean, just rinse most of the large goopy stuff away.
Get the garden hose and run it into the window and into the tank. Turn on the faucet just long enough that water starts into the tank. (it's not a lot of chlorine in a large tank so don't worry)
Turn off the water and unhook the hose from the faucet. What you have done is "primed" the hose and it will begin to siphon. Yeah, this works great and fast since hoses are big in diameter.
It's pretty easy overall. I have a 30 gallon and a 55 gallon tank, and only have to clean them about 2 times a year. I have no gravels and excellent water circulation, so all the waste gets to the filter. For the most part, all I have to do is water changes.