Whenever we wash the clothes lately - our toilet water goes down, our pan for our water heater fills up with foul smelling water & our shower bubbles with water. We were told we need to run a hose from our washer to the outside. How would we go about doing that?

That may violate code. Maybe you need to rent a drain auger and auger your drains starting where the last thing you are having problems with connects in. If you house is new enough to have a clean out near where the drain exits the house, run the auger down it too.

Would like to add that labman is also correct on code. I can't speak for every state, but in Virginia it is, and if the health department finds out your draining a washing machine outside, they will make you correct it.

Although it doesn't make much sense to me. I personally feel like keeping wash water out of the septic is better for the system.