When I flush the toilet, take showers in my upstairs bathroom, water comes out of my downstairs shower. How can I fix this problem without calling a professional plumber?

When I flush the toliet, take showers in my upstairs bathroom, water comes out of my downstairs shower. How can I fix this problem without calling a professional plumber?

You have a clog downstream from the downstairs bathroom that nust be snaked. Are you on city sewer or a septic tank?
First you must rent a sewer snake,(see image) ( home Depot rents them)
There are several ways to do this.
1) Pull the downstairs toilet and snake from there, **or**
2) Locate the dowhstairs lavatory roof vent and snake from there.
If you snake from downstairs put out 20 feet of snake and if you snake from the roof put out enough snake to reach the base and 20 feet more.
Let me know about the septic tank. Regards, T^om