First you have to take out the turtle(s). But be careful taking out a turtle can be a dangerous task. A turtle bite is painful and the don't let go. So try grabing the turtle from its side, and not put your fingers in front of the turtle.
Before going on put the turtle(s) in a container with atleast 18 ounces of water, in order to keep the turtle(s) hydrated. Next unplug the filter and take it out of the tank with any other decorative objects.
Step 2: Cleaning
It is preferable to clean your tank outside, debris and shedded shells can clog up your sink or bathtub if cleaned there. Ok so the next step is to pour the water out. With a piece of cloth try taking any extra shedded shell or debris from the tank before continuing. Next rinse down the tank in order to take out in trash stuck on the walls of the tank. That is all really need to clean, but if chemiclas are being used to clean for example, the glass, then remember to rinse out the take about 2-4 times. Chemical left in the tank can make your turtle sick and even kill it. Wash any decorative items. With another peace of cloth or paper towels dry the tank up, and take it inside.
Once place in the desired spot, fill the tank about 2/5 full.(preferably mild temperature water, high/freezing temperatures can kill the turtle). Some turtle's shell is to big and won't be able to swim to the surface for air if filled with more the half of the tank full (turtle tanks are usually atleast hold 20 gallons). Next put the filter, and decorative items back in the tank. Before putting the turtles back in though it is recommended to use a water conditioner. The water conditioner helps remove chlorine, chloramines, ammonia, adds electrolytes such as calcium, and reduces the pH of the water. if used let it sit and filter in the water for about 30 minutes. Next put the turtles back in, and your turtle is ready to be happy!