The dishwater will back up with water and then overflow on the floor when I am not even using it.

Then you don't have a proper air gap and the discharge from the sink is draining into it. Go back and run a high loop air gap, (see image) Install the drainage hose with a large loop between the point where it discharges from the dishwasher and enters the sink drain. Think of a bell-shaped curve. Make sure to attach the drain hose as far up in the cabinet as you can, using hose clamps or a pipe strap to secure it, when you run it between the dishwasher and the disposal. Good luck, Tom

I agree completely with Tom. I also think you may have a partiall blockage in your sink drain causing water to reach the level of your dishwasher tie-in. I would suggest augering your sink drain, and doing the high loop for the d.w. drain as Tom suggested.