If a waste line is supposed to enter a stack through a Sani-T, and a Sani-T is a 90 degree angle, how are you supposed to maintain the slope of the waste pipe between the toilet and the stack?

I mean, the waste line is vertical out of the bottom of the toilet, enters a 90 degree bend and then travels until it hits a 90 degree Sani-T at the stack. How are you supposed to maintain any type of slope if all you can work with are right angles?